JP3999344B2 - Hot-rolled steel strip for building structure with less cracking in scale and excellent corrosion resistance, and its manufacturing method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel material used as a structural member of a detached house, an apartment house, a large building, a building, a bridge, or the like, and a method for manufacturing the same.
[0002]
[Prior art]
Due to stricter building safety standards and the pursuit of functionality, steel materials for pillars and beams are required to have higher functionality. In particular, corrosion resistance is an important factor that affects the service life of a structure. Improvement of its properties is required, and stainless steel for building structures that has been free from rust has attracted attention.
[0003]
For structural use, SUS304 (18Cr-8Ni), which has excellent corrosion resistance and toughness, has many uses. However, since stainless steel requires a large amount of expensive elements such as Cr and Ni, the material cost and the manufacturing cost are high, and although it is functionally superior, there is a problem in economic efficiency. Thus, although rust and corrosion are inevitable, steel materials are being developed that are inexpensive, suppress the progress of corrosion, and minimize the amount of corrosion.
[0004]
For example, in JP-A-60-162507, the surface scale of a slab of ordinary steel is removed, glass paper is attached and rough-rolled, and then the glass melt film is removed and finish-rolling is performed. And a method for producing a black skin scale film having excellent corrosion resistance is disclosed.
[0005]
Japanese Patent Application Laid-Open No. 8-199289 discloses a steel containing 0.50 to 1.50% Cr, and an oxide scale having a thickness of 10 μm or less having a chromium oxide film between the base material and the scale in a hot rolling process. An H-section steel having is disclosed.
[0006]
[Problems to be solved by the invention]
However, the invention described in Japanese Patent Application Laid-Open No. 60-162507 increases the number of processes accompanying the attachment and peeling of glass paper, inevitably decreases the production capacity, and increases the manufacturing cost. Sex remains a problem.
[0007]
The invention described in JP-A-8-199289 is a technique for forming an oxide layer having excellent corrosion resistance on the surface of ordinary steel that has been inferior in corrosion resistance. Therefore, if corrosion progresses through the oxide layer, the effect of improving corrosion resistance is lost, so it is impossible to maintain a low corrosion rate below that of ordinary steel for a long time in a humid environment. It is impossible to improve the long-term durability of objects.
[0008]
By the way, when considering a structural material such as a pillar or beam of a building, the corrosive environment is roughly divided into two periods after the exterior material is completed. The latter is not so severe as a corrosive environment because free inflow of outside air is regulated by the exterior material and interior material. Rather, although the time is short, the corrosiveness of the environment is more severe in the former period where it is directly exposed to wind and rain or dust. Further, if rust is generated on the surface of the steel material before the construction of the exterior material is finished, there is a problem that corrosion is likely to proceed under the rust layer even after the exterior material is formed. That is, it is the corrosion resistance against the occurrence of rust that dominates the durability of the structural material. Therefore, the most economical method for imparting corrosion resistance to the structure is most effective in improving only the anticorrosion performance during processing and construction of the material.
[0009]
As an initial rust prevention treatment for general steel materials, applying rust prevention oil is widely performed. However, simply applying rust-preventing oil to the hot-rolled steel strip where scales are present or hot-rolled pickling plates from which scales have been removed by pickling, the oil film is easily removed by mechanical contact with foreign matter. Therefore, it is impossible to provide sufficient corrosion resistance at the time of processing such as drilling or bending of steel material performed at the factory prior to construction or at the actual construction stage. Therefore, in order to improve the rust resistance during the construction period at a low cost, it is necessary to maintain the effect of the rust preventive oil against contact with foreign matter and wrinkles.
[0010]
In order to maintain the effect of this rust-preventing oil, the present inventor aims to develop a hot-rolled steel strip that does not crack in the scale during processing during construction, etc., and the metal surface is not exposed. Detailed investigations have led to the present invention.
[0011]
[Means for Solving the Problems]
The gist of the present invention is as follows.
In order to have corrosion resistance during the construction period for building structures,
(1) In mass %,
C: 0.005% to 0.1%, Si: 0.3% to 1.5%,
Mn: 0.05% to 1.5%, P: 0.04% or less,
S: 0.05% or less, N: 0.055 or less, Cr: 10.58-15 %, and C + N: 0.1% or less,
The balance is composed of Fe and inevitable impurities, and SiO ₂ is formed into a film or lump with a thickness of 0.1 μm to 1 μm from the scale to the base metal. Hot-rolled steel strip for construction.
[0012]
(2) In mass %,
Mo: 0.1-2.5%, Cu: 0.1-2.5%,
Ni: 0.1 to 2.5%
(1) The hot-rolled steel strip for building structures with less cracking in the scale and excellent corrosion resistance.
[0013]
(3) After the slab is reheated to a temperature range of 1100 to 1300 ° C., the hot rolling is finished at 800 ° C. or higher, the steel sheet is wound at 550 ° C. or higher, and then held at 550 ° C. or higher for 34 minutes or longer. A method for producing a hot-rolled steel strip for a building structure that is less likely to generate cracks in the scale as described in (1) or (2) and has excellent corrosion resistance.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be described in detail below.
As described above, in order to allow rust prevention oil to exist stably during construction, in the case of a hot-rolled steel strip with scale, the scale does not peel off during processing, etc., and rust prevention oil is not applied. It is important not to expose the metal surface. As a method for producing a hot-rolled steel strip that satisfies this requirement, the present inventors have studied a technique for concentrating effective elements at the interface between the scale and the underlying metal in the hot-pressing step to remarkably improve the adhesion.
[0015]
Accordingly, a ferritic stainless steel having a Cr content of 12% is used as a base, and steels with different amounts of addition of various elements such as Mn and Si are manufactured. After heating in the range of 1100 to 1300 ° C, 800 ° C or higher The hot-rolled steel strip was manufactured by ending hot rolling at 550 ° C. or higher. These are subjected to a 90 ° bending test, and then a cello tape is applied to the inner surface side to which the compressive stress is applied and the outer surface side to which the tensile stress is applied, and then the scale area ratio remaining on the remaining steel surface is peeled off (the steel plate surface). The remaining area of the scale per unit area was determined.
[0016]
Among various additive elements, Si has a remarkable effect on adhesion, and the result is shown in FIG. When the Si content was 0.3% or more, the scale remaining area ratio was 100%, and there was obtained a hot-rolled steel sheet excellent in adhesion with no scale peeling and no cracks in the scale. In order to clarify this cause, the scale and base metal interface of hot-rolled steel strips with different amounts of Si are observed with an optical microscope, and the concentrated state of Si is further detected with an EPMA and Auger electron spectrometer, followed by X-ray diffraction. The existence state of Si was analyzed in detail. As a result, the inner oxide of SiO ₂ in the vicinity of the interface of the underlying metal by addition of Si is formed, only it is less than 0.3% distributed in dots at the interface, even 0.1μm or less the average thickness of the SiO ₂ Met.
[0017]
On the other hand, in the steel added with 0.3% or more of Si in which scale peeling did not occur even when bending at 90 °, SiO ₂ was densely present in the form of a film or a lump at the interface between the scale and the base metal, and the thickness was also 0.1%. It was about 1 μm to 1 μm. Although details about the details of adhesion improvement by SiO ₂ are unclear,
(1) When SiO ₂ grows to about 0.1 μm to 1 μm, the unevenness of the metal interface increases, and the adhesion is remarkably improved by the anchor effect.
[0018]
(2) It is considered that this is because SiO ₂ grows so as to extend over both the metal phase and the scale phase, thereby further increasing the bonding force between the scale and the base metal. In any case, when Si is added at 0.3% or more and manufactured at the above-mentioned heating temperature, hot rolling temperature, and scraping temperature, the new knowledge that it is possible to prevent cracks due to processing as well as scale peeling. I came to get.
[0019]
Further, as an advantage of the accompanying SiO ₂ formation, even if the hot-rolled steel strip to which Si is added is pickled, rust prevention oil is applied, and then the surface of the pickled plate is dazed with a tool. It was confirmed that rust-preventing oil wraps around the area and that the heel can be repaired with rust-preventing oil. This is presumably because the rust prevention oil that existed in the recesses around the buttock wraps around the ridges due to the increase in the irregularities on the metal surface due to the formation of SiO ₂ .
[0020]
Next, the component range of the molten steel of the present invention and the reason for limiting the manufacturing method will be described.
C is an effective element for improving the strength of steel. However, if it is less than 0.005%, the strength required for structural steel cannot be obtained. Moreover, the excessive addition exceeding 0.1% reduces remarkably the base material toughness 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%.
[0021]
Si needs to be added to molten steel as a deoxidizer in order to reduce the dissolved oxygen in the steel and ensure hot workability. However, if it is less than 0.3%, there is no effect in improving the adhesion of the scale of the hot-rolled steel strip as described above. On the other hand, adding over 1.5% impairs the toughness of the base metal and the welded portion. The lower limit was 0.3% and the upper limit was 1.5%.
[0022]
Mn needs to be added to molten steel as a deoxidizer and desulfurizer. If it is less than 0.05%, a predetermined effect cannot be obtained. On the other hand, if added over 1.5%, the toughness and cracking properties of the base metal and the welded part are impaired, so the lower limit was made 0.05% and the upper limit made 1.5%.
[0023]
When P is present in a large amount, not only the weldability is impaired, but also a phenomenon that promotes the generation of rust appears. Therefore, it was limited to 0.04% or less.
[0024]
S is not only a starting point for rust as a sulfur-based inclusion such as MnS, but also increases the corrosion rate. Furthermore, it segregates at the grain boundaries and harms hot workability. Therefore, it is necessary to regulate to 0.05 %% or less. S is more preferable as an impurity.
[0025]
N is an unavoidable impurity element and is effective in improving the strength of steel, but excessive addition exceeding 0.055% hardens the martensite phase and significantly reduces the toughness of the heat affected zone. And cracking may occur during welding. The upper limit was made 0.055%.
[0026]
Cr has the effect of suppressing the occurrence of corrosion and reducing the corrosion rate in the atmospheric environment. In particular, regarding the suppression of rust generation, it exhibits a synergistic effect with rust prevention oil impregnated in oxide scale. Also, once corrosion occurs and a rust layer is formed, it has the effect of reducing the overall corrosion rate of the steel material under the rust layer. However, when the amount of Cr added is less than 10.58 %, the effect of weakening rust generation and reducing the corrosion rate is weak. On the other hand, if added excessively, raw material costs and smelting costs increase, so the economic efficiency decreases. From the above points, the lower limit was set to 10.58 % and the upper limit was set to 15%.
[0027]
Mo, Cu, and Ni have the effect of suppressing the occurrence of corrosion and reducing the corrosion rate in the atmospheric environment, like Cr. Therefore, when the effect of improving the corrosion resistance by adding Cr is weak, it is necessary to add one or more of Mo, Cu, and Ni. However, if the amount is small, the effect is weak, and if it is added excessively, raw material costs and manufacturing costs increase, and the economic efficiency decreases. Therefore, the lower limit is set to 0.1% and the upper limit is set to 2.5%.
[0028]
By the way, the formation of the Si oxide in the present invention cannot be effectively performed only by restricting the chemical components in the steel to the above-described range and performing hot rolling by a normal method. In order to exhibit such unprecedented characteristics, it is necessary to control the properties of the steel sheet metal surface and the generated oxide scale by the manufacturing method described below, in addition to the chemical composition of the steel.
[0029]
In the present invention, first, molten steel having a chemical composition in the above-described range is cast, and then reheated to a temperature range of 1100 to 1300 ° C. When the heating temperature is less than 1100 ° C., the amount of SiO ₂ formed during heating is small, and the subsequent hot rolling process does not grow sufficiently, and SiO ₂ having a thickness of 0.1 μm or more cannot be present at the interface. Further, when the heating is less than 1100 ° C., the rolling reaction force during the subsequent hot rolling is increased, which is also a problem in manufacturability. On the other hand, the upper limit was set to 1300 ° C. from the performance and economy of the heating furnace.
[0030]
The heated slab is controlled to a predetermined thickness by rough rolling and finish rolling, but the finish rolling finish temperature is 800 ° C. or more from the point of promoting the growth of SiO ₂ effective for adhesion between the scale and the base metal. However, the finish exceeding 1000 ° C. is preferable because the crystal grains of the steel sheet are coarsened and the toughness required as a structural material is lowered.
[0031]
Furthermore, since the higher the cutting temperature after finish rolling is, the higher the effect of SiO2 growth is, the higher the finish rolling temperature is, the temperature is set to 550 ° C. or higher. Further, after the scraping, in order to stabilize the scale properties by transformation of the Fe-based oxide formed on the hot-rolled sheet and the composite oxide of Fe and Cr, it is necessary to maintain for 34 minutes or more.
[0032]
【Example】
Hereinafter, the present invention will be described in detail based on examples.
[0033]
The prototype steel shown in Table 1 was melted in a converter and cast into a slab by continuous casting. Then, after reheating in a heating furnace, it was rolled to a thickness of 3 mm with a rough rolling mill and a finish rolling mill and wound into a coil. Thereafter, the coil was put in a heat insulation furnace and kept at a predetermined temperature. Moreover, the manufactured hot-rolled steel strip was pickled with a normal pickling line.
[0034]
Rust prevention oil was applied by brush and dried at room temperature for 3 hours. As the rust-preventing oil used, solvent diluted rust-preventing oil Type 3 No. 1 described in JIS K 2246 was used.
[0035]
The rust resistance was evaluated by the following method. The steel material cut into a size of 100 mm × 70 mm was coated with rust preventive oil by brush and dried at room temperature for 3 hours. Thereafter, 45 ° to 135 ° bending at 1 t or 2 t was performed, and then a 5% NaCl aqueous solution heated to 35 ° C. was sprayed for 100 hours. The degree of rusting was observed with the naked eye. In addition, an experiment was carried out in which a cocoon was put in a cemented carbide tool after bending and a 5% NaCl aqueous solution heated to 35 ° C. was sprayed for 100 hours, and the rust generation state in the heel was observed with the naked eye.
[0036]
No. of the present invention. 1 to 3 and 5, almost no rust was observed. In 9 to 13, rust was remarkably generated.
[0037]
[Table 1]
[0038]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the steel material excellent in corrosion resistance which fully utilized the effect of the rust prevention oil used as a structural member of buildings, such as a detached house, an apartment house, a large building, a building, and a bridge, can be supplied at low cost. It becomes possible.
[Brief description of the drawings]
1 is a diagram showing the relationship between the scale peeling area of Si amount and 90 ° C. bend after the test.

Claims (3)

% By mass
C: 0.005% to 0.1%,
Si: 0.3% to 1.5%
Mn: 0.05% to 1.5%,
P: 0.04% or less,
S: 0.05% or less,
N: 0.055 or less,
Cr: 10.58-15 %, and C + N: 0.1% or less,
The balance is composed of Fe and inevitable impurities, and SiO ₂ is formed into a film or lump with a thickness of 0.1 μm to 1 μm from the scale to the base metal. Hot-rolled steel strip for construction. % By mass
Mo: 0.1 to 2.5%,
Cu: 0.1 to 2.5%,
Ni: 0.1 to 2.5%
The hot-rolled steel strip for building structures with less cracking in the scale and excellent corrosion resistance according to claim 1, further comprising at least one of the following. After the slab is reheated to a temperature range of 1100 to 1300 ° C., the hot rolling is finished at 800 ° C. or higher, the steel sheet is wound up at 550 ° C. or higher, and then kept at 550 ° C. or higher for 34 minutes or longer to be cooled. The method for producing a hot-rolled steel strip for a building structure that is less likely to generate cracks in the scale according to claim 1 or 2 and has excellent corrosion resistance.