JP3039630B2 - Low corrosion rate steel plate with low local corrosion - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low corrosion rate steel sheet which is less likely to cause local corrosion and is used as a structural member of a house such as a door-to-door house, an apartment house, a large building, a building or a bridge.

[0002]

2. Description of the Related Art Due to improvements in durability of buildings and stricter safety standards, materials for beams and columns are required to have even higher corrosion resistance. In particular, public buildings are a state property that plays a part in social capital, and must have long-term durability of at least 100 years or more. As a manifestation of such social demands, there is a movement to use stainless steel as structural steel in addition to conventional ordinary steel. For example,
Research and development for applying stainless steel to structural members such as columns and beams was conducted between 1988 and 1992 as part of the Ministry of Construction's comprehensive development project. As a result, SUS304 (18Cr-8Ni) has obtained general building certification from the Minister of Construction.

[0003] However, the above SUS304 is made of Cr, N
Since a large amount of an expensive alloy element such as i is required, the production cost is higher than that of ordinary steel, and although its function is excellent, there is a problem in its economic efficiency. However, when the added amount of Cr or Ni is reduced, pitting or crevice corrosion occurs in a humid environment. When such local corrosion occurs, unlike the case where the entire surface is uniformly eroded, the load is concentrated on the portion where the plate thickness is reduced due to the erosion, and the strain and deformation are locally concentrated. In the worst case, buckling or breakage occurs at the site where local corrosion occurs. Because corrosion is local corrosion,
Erosion in the depth direction (thickness direction) is faster than overall erosion,
Inadvertent reduction of Cr and Ni significantly reduces durability and reliability as a structural material. In addition, it is difficult to predict in which part of the material the local corrosion will occur, which makes it difficult to design the strength of the whole building and lowers its reliability.

On the other hand, if the form of corrosion is not local corrosion but overall erosion, deformation and strain when a load is applied are uniform, and problems such as breakage and buckling are caused by folding the corrosion margin into the design. Can be avoided. Furthermore, if the overall corrosion rate is lower than that of conventionally used ordinary steel or weather-resistant steel, it can be used as a structural material for buildings for a longer period of time. Therefore, in order to improve the durability of a building, it is inevitable that corrosion and rust are generated over the entire surface, but a steel material that is inexpensive and suppresses the progress of corrosion is required.

[0005] Techniques for making such overall erosion inevitable and reducing the corrosion rate have been developed as techniques for improving the corrosion resistance of ordinary steel. For example, Japanese Patent Application Laid-Open No. 60-162507 discloses a method in which the surface scale of a steel slab is removed, glass paper is adhered, rough-rolled, removed, and finish-rolled, and black is excellent in adhesion and corrosion resistance. A method for making a leather scale coating is disclosed. However, in this method, the number of processes increases with the attachment and detachment of the glass paper, and these process capabilities are generally the lowest in a production line and limit the overall production capability. As a result, the production capacity of the entire process is reduced, and the production cost is increased. As a result, this method has a problem in economical efficiency although the corrosion resistance is improved.

Japanese Unexamined Patent Application Publication No. 8-199289 discloses that in a hot rolling step of steel containing 0.50 to 1.50% of Cr, a chromium oxide film having a thickness of 10 μm or less having a chromium oxide film between a base material and a scale is provided. H-section steels having an oxide scale of However, the present invention is a technique for forming an oxide layer having excellent corrosion resistance on the surface of ordinary steel having originally poor corrosion resistance. Therefore, when corrosion progresses through the oxide layer, the effect of improving corrosion resistance is lost. Therefore, in this method, it is impossible to maintain a low corrosion rate equal to or lower than that of ordinary steel for a long time in a humid environment, and it is impossible to improve the long-term durability of a building.

[0007]

SUMMARY OF THE INVENTION The present invention relates to a steel plate which dramatically improves the service life of a building structure while maintaining economic efficiency, and more particularly to a steel plate which suppresses the occurrence of local corrosion and has a corrosion rate of general erosion. However, the present invention provides an inexpensive low corrosion rate steel sheet having a service life of 100 years or more as a structural material even when used without painting, being extremely lower than ordinary steel.

[0008]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and the feature of the present invention is to provide a steel base material having a corrosion rate reduced by alloying of Cr,
By forming a chromium-deficient layer with a lower Cr content than the base material, corrosion is generated entirely, and the growth rate of subsequent overall erosion under the rust layer is suppressed to make local corrosion less likely to occur. is there. The gist of the present invention is the following (1) to (3) .

(1) By weight%, C: 0.005 to 0.1
%, Si: 0.05-1.5%, Mn: 0.05-1.
5%, P: 0.1% or less, S: 0.05% or less, Cr:
A steel sheet containing 6 to 18%, with the balance being Fe and unavoidable impurities, characterized in that an oxide scale and a chromium-depleted layer with a thickness of 5 μm or more are formed immediately below the oxide scale on the surface layer of the steel sheet. Low corrosion rate steel sheet that is unlikely to cause local corrosion.

(2) The low corrosion rate steel sheet according to the above (1), wherein the oxide scale on the surface layer of the steel sheet is removed, and the chromium-depleted layer is exposed on the surface of the steel sheet. Corrosion rate steel plate.

(3) The low rot of (1) or (2) above
Cr concentration in the chromium-deficient layer in a corrosion rate steel sheet
Is 2% or more lower than the Cr concentration of the steel sheet base
Low corrosion rate steel sheet with low local corrosion.

Incidentally, the oxide scale and the chromium-deficient layer referred to in the present invention have the definitions shown in FIG. That is, changes in the concentrations of Cr and O from the steel sheet surface in the depth direction are analyzed by glow discharge optical emission spectroscopy (GDS), and the maximum concentration of oxygen near the surface is OF, and the oxygen concentration of the substrate is OM. The depth at which the oxygen concentration becomes (OF + OM) / 2 is defined as the interface between the oxide film and the metal. Then, the metal C on the metal side from this interface
A region where the Cr content is lower than the r concentration is defined as a chromium-deficient layer. Further, when the oxide film has a thickness of 0.05 μm or more, this is defined as an oxide scale. On the other hand, when the oxide film has a thickness of less than 0.05 μm, it is defined as a film formed in the air, and when a chromium deficient layer exists on the steel sheet surface via the film formed in the air as described later, this chromium The depletion layer is exposed on the steel sheet surface. In order to cause overall corrosion, it is only necessary that a chromium-depleted layer is present. The Cr concentration in the chromium-depleted layer is not particularly specified, but is preferably 2% lower than the Cr concentration of the substrate. It is preferable that it is reduced as described above.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the composition range of the steel of the present invention will be described below. C is an element effective for improving the strength of steel. If it is less than 0.005%,
The required strength for structural steel cannot be obtained. Further, an excessive addition exceeding 0.1% significantly lowers the toughness of the base metal and the toughness of the heat affected zone. Therefore, the lower limit is set to 0.005% and the upper limit is set to 0.1%.

Si must be added to the molten steel as a deoxidizing agent in order to reduce solid solution oxygen in the steel and ensure hot workability. If it is less than 0.05%, the deoxidizing effect is weak. On the other hand, 1.5
%, The lower limit is set to 0.05% and the upper limit is set to 1.5%. Mn
Must be added to molten steel as a deoxidizing agent and a desulfurizing agent. If it is less than 0.05%, the desired effect cannot be obtained. On the other hand, if added in excess of 1.5%, the toughness and cracking properties of the base metal and the weld are impaired, so the lower limit is 0.05% and the upper limit is 1.5%.
%.

If P is present in a large amount, it impairs weldability.
Therefore, it is necessary to regulate to 0.1% or less. P is preferably as small as an impurity. S mainly serves as a starting point of local corrosion as a sulfur-based inclusion such as MnS, and also causes an increase in the corrosion rate of general corrosion. Further, they segregate at grain boundaries and impair hot workability. Therefore, 0.
It is necessary to regulate to less than 05%. S is preferably as small as possible at the impurity level.

Cr has the effect of reducing the rate of general corrosion of a steel material under a rust layer in a wet environment. This effect is not exhibited by the chromium oxide concentrated in the rust layer, but is exhibited by the metal Cr in the steel material. However, if the added amount of Cr is small, the desired effect cannot be obtained. On the other hand, if added excessively, a passive film having excellent corrosion resistance is formed immediately below the oxide scale and under the rust layer, so that local corrosion such as pitting and crevice corrosion is caused. Occurs. Therefore,
The lower limit was 6% and the upper limit was 18%.

If the chemical composition of the steel is restricted to the above-mentioned range, it is not possible to obtain a low-corrosion-rate steel which is less likely to cause local corrosion and has a low overall corrosion rate. In order to exhibit such unprecedented properties, it is necessary to control the properties of the steel sheet surface in addition to the chemical composition of the steel. In the present invention, the reason why the steel sheet has an oxide scale and a chromium-depleted layer having a thickness of 5 μm or more immediately below the oxide scale on the surface layer portion is as follows: (1) suppression of local corrosion occurrence; (2) low overall corrosion rate; 3) To achieve both economic efficiency.

First, the oxide scale is harmful for suppressing the occurrence of local corrosion.
In order to reduce the production cost of the steel sheet and secure the economic efficiency, it is necessary that the oxide scale generated in the course of heating / rolling and subsequent winding remain as it is. However, even if the oxide scale exists, the thickness of 5 μm
The presence of the chromium-deficient layer makes it possible to stably maintain the state of overall corrosion. The chromium-depleted layer is required to be eroded preferentially in the early stage of corrosion, and to cause general corrosion. In the case of the chromium-deficient layer immediately below the oxide scale, if the thickness is small, it is difficult for the overall corrosion to occur. Therefore, the thickness of the chromium-deficient layer is set to 5 μm or more. The upper limit of the thickness is not particularly limited, but it is economically desirable that the corrosion margin is as small as possible.
m or less is preferable.

The oxide scale on the surface layer of the steel sheet, which is a feature of the low corrosion rate steel sheet of the present invention , and
The chromium-depleted layer having a thickness of 5 μm or more immediately below
The continuous cast slab of the chemical composition in
After hot rolling, finish rolling at 800-1000 ° C
And wind the obtained steel sheet into a coil at 350 ° C. or higher.
Can be obtained by: This is the finish rolling temperature
Is 800-1000 ° C and the winding temperature is 350 ° C or more
, Thick oxide scale is formed on the surface of the steel sheet,
Under the oxide scale, thick and low in Cr concentration
This is because a chromium deficiency layer is generated. 1000 ° C
After finishing rolling, the grain size of the steel sheet becomes coarse
In addition, the required toughness as a structural material decreases, and
If the temperature is lower than 00 ° C., the rolling process is difficult.

The higher the temperature, the greater the thickness of the surface layer of the steel sheet.
Oxidized scale is generated and directly below the oxidized scale
, A thick chromium-depleted layer with low Cr concentration
Therefore, there is no upper limit to the winding temperature, but when the temperature is extremely high, a chromium deficiency layer is formed after winding, but the diffusion rate of Cr from the substrate to the chromium deficiency layer increases , and the chromium deficiency tends to recover. since appears, in order to efficiently form a chrome-depleted layer is preferably wound at 780 ° C. or less.

In the case where a chromium deficient layer is provided immediately below the oxide scale, the oxide scale existing on the chromium deficient layer may be peeled off when the general corrosion grows to some extent. For this reason, the periphery of the pillar or the beam is stained, which is not preferable in terms of design. In this case, although the production cost of the steel plate increases, it is necessary to remove only the oxide scale and expose the chromium-depleted layer on the surface of the steel material. When the chromium-depleted layer is exposed, general corrosion occurs regardless of its thickness. Here, the state in which the chromium deficient layer is exposed refers to a state in which part or all of the oxide scale covering the chromium deficient layer at the time of generation of the chromium deficient layer is peeled off as defined above. After the oxide scale is peeled off, a film formed in the air formed by the chromium-deficient layer coming into contact with the air may be present on the surface.

As described above, in order to remove the oxide scale and expose the chromium-depleted layer, the surface of the unwound steel sheet is subjected to a shot blast treatment. This is because this method can expose the chromium deficient layer relatively inexpensively and stably. The pickling treatment cannot be applied because the chromium-depleted layer is also dissolved in the process of removing the oxide scale.

[0023]

EXAMPLES The prototype steels shown in Table 1 were melted in a converter and cast into slabs by continuous casting. Then, after reheating in a heating furnace, the resultant was rolled to a thickness of 3 mm in a rough rolling mill and a finishing rolling mill, and then air-cooled and water-cooled as necessary, and wound into a coil. In addition, a part was subjected to shot blasting on both sides of the steel plate. The above processing conditions were as shown in Table 1.

[0024]

[Table 1]

[0025]

[Table 2]

The obtained steel sheet was evaluated for the presence or absence of an oxide scale and a chromium-depleted layer by analyzing changes in the concentrations of Cr and O from the steel sheet surface in the depth direction by glow discharge emission spectroscopy (GDS). Then, based on FIG. 1, the thickness of the chromium deficient layer and whether the chromium deficient layer was exposed were measured and determined. Next, the corrosion resistance was evaluated by a combined cycle corrosion test. Conditions are (1) artificial seawater spray (35
(4 hours at 60 ° C), (2) drying (2 hours at 60 ° C), (3)
The combination of wetness (50 ° C., relative humidity of 95% or more, 2 hours) was defined as one cycle, and the evaluation was made based on the difference between the maximum erosion depth and the average erosion depth after 30 cycles. The smaller the difference between the two, the less local corrosion occurs. Therefore, (maximum erosion depth) / (average erosion depth) was evaluated as the local corrosion degree. The maximum erosion depth is determined using an optical microscope.
The average pit depth was measured from the weight loss of the specimen. In addition, both the maximum erosion depth and the average erosion depth were measured after removing rust with 2 ammonium citrate (boiling). As for the local corrosion degree, 2 or less was determined to be good.

The mechanical properties were determined to be 0.1 by a tensile test.
The 1% proof stress was evaluated for the absorbed energy vE20 by an impact test. The former was tested in the L direction of a JIS No. 13 B test piece, and the latter was tested in the L direction of a 3 mm thick sub-sized JIS No. 4 test piece. 0.1% proof stress is 250-350N / m
The range of m ² and the impact value of 60 J or more were judged to be good. Table 2
The results of evaluating the corrosion resistance are shown in FIG. First, the steels of the present invention Nos. 1 to 23 all have an oxide scale and a chromium-deficient layer having a thickness of 5 μm or more immediately below the oxide scale on the surface. Among these, the average erosion depth decreases as the amount of Cr added to the base increases. Furthermore, the local corrosion degree is 2 or less, indicating that the maximum erosion depth is twice or less of the average erosion depth, and has a property that local corrosion hardly occurs. Nos. 24 and 25 are obtained by subjecting the steel plate of No. 3 to shot blasting to expose the chromium-depleted layer on the surface. Compared with the steel sheet of No. 3, the local corrosion degree is reduced, and it has excellent durability and reliability as a structural material.

On the other hand, in the comparative steel No. 26 containing Cr beyond the range of the Cr addition amount of the present invention, the average erosion depth is shallow, but the local corrosion degree is 2.94, and local erosion is apt to occur. . Similarly, in Nos. 27 to 29 containing only Cr less than the range of the Cr addition amount of the present invention, both the average erosion rate and the local corrosion degree are large, and the steel sheet of the present invention (No. 1)
6) As compared with 6), it is inferior in reliability and durability as a structural material.

Numbers 30 to 37 indicate that one of the components other than Cr is outside the scope of the present invention. Number 30 with less C
No. 31 having low strength and number 31 having a large amount of C have high strength and low toughness and are not suitable for structural steel sheets. Numbers 32 to 35 in which Si or Mn are out of the range of the present invention also have high strength and low toughness. In the case of No. 36 in which P was added beyond the range of the present invention, the strength and toughness were in the appropriate ranges, but cracks occurred during arc welding. No. 37 in which S was added beyond the scope of the present invention
Shows that the corrosion rate is much higher than that of the steel type having the same Cr content. Furthermore, this test material has ear cracks during rolling, and has extremely poor hot workability.

Next, No. 38 (finish rolling temperature: 750 ° C.)
And No. 39 (winding temperature 250 ° C) is finish rolling
Temperature and winding temperature of the low corrosion rate steel sheet of the present invention .
Finish rolling temperature in manufacturing method (800-1000 ° C)
And the temperature deviates from the winding temperature (350 ° C or higher).
As a result, it is a comparative steel in which the thickness of the chromium-depleted layer is less than 5 μm.
In this comparative steel, since the thickness of the chromium-depleted layer is less than 5 μm, the average erosion depth is the same as that of the steel of the present invention, but local corrosion exceeds 2, and corrosion is likely to occur locally.
No. 40 is obtained by removing the oxide scale and the chromium deficient layer generated in the hot rolling step by mechanical grinding. Although the Cr content in the steel is within the range of the present invention, since there is no chromium-depleted layer, the local erosion degree is large.

[0031]

[Table 3]

[0032]

[Table 4]

[0033]

Industrial Applicability According to the present invention, a structural steel sheet which is less likely to cause local corrosion and has a low overall erosion rate by utilizing a chromium deficient layer generated on the steel material surface by an oxidation reaction in hot rolling and a subsequent cooling process. It can be provided, which contributes to the improvement of long-term reliability of buildings, safety assurance, and economic efficiency.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing the state of formation of an oxide scale and a chromium deficient layer on the surface layer of a steel sheet.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 7/06 C21D 8/02

Claims (3)

(57) [Claims] C: 0.005 to 0.1%, Si: 0.05 to 1.5%, Mn: 0.05 to 1.5%, P: 0.1% or less, by weight% S: 0.05% or less, Cr: it includes 6 to 18% the balance being a steel sheet consisting of Fe and unavoidable impurities, the thickness from the surface oxide scale, and <br/> immediately below at its steel plate surface layer portion A low corrosion rate steel sheet in which local corrosion does not easily occur, wherein a chromium deficiency layer having a thickness of 5 μm or more is formed. 2. The low corrosion rate steel sheet according to claim 1, wherein the oxide scale on the surface of the steel sheet is removed and a chromium-depleted layer is exposed on the surface of the steel sheet. steel sheet. (3) The low corrosion rate steel sheet according to claim 1 or 2.
In the above, the Cr concentration in the chromium-deficient layer is
It is characterized by lowering by more than 2% from the Cr concentration in the ground
Low corrosion rate steel plate with low local corrosion.