JP4507668B2 - Manufacturing method of high corrosion resistant steel - Google Patents

Description

  The present invention relates to a method for producing a weather resistant steel used for an outdoor steel structure, and in particular, a weather resistant steel (hereinafter referred to as a high corrosion resistant steel) that significantly improves corrosion resistance and suppresses the occurrence of flow rust. It relates to a manufacturing method.

  Weathering steel widely used for outdoor steel structures (for example, bridges) is improved in corrosion resistance in the atmosphere by adding alloy elements such as P, Cu, Cr, Ni and the like. Needless to say, the cause of corrosion of steel materials outdoors is oxygen and water. Weatherproof steel forms rust that is difficult to pass oxygen and water (hereinafter referred to as protective rust) on the surface, and suppresses the steel from coming into contact with oxygen and water, thereby suppressing the progress of corrosion. It takes several years to form the protective rust, but after the protective rust is formed, the progress of the corrosion is remarkably slowed. Therefore, it is not necessary to apply a rust preventive coating to the weather resistant steel.

  On the other hand, a steel structure using ordinary steel materials must be coated with a rust preventive paint, and the coating of the rust preventive paint deteriorates when exposed to wind and rain, so it is necessary to repaint regularly. .

  Since the steel structure using weathering steel does not require the application of a rust-preventive paint or periodic repainting, the maintenance cost of the steel structure can be reduced.

  However, since the effect of preventing corrosion is inferior for several years until the protective rust is formed, a rust (hereinafter referred to as a flow rust) that appears as if it has flowed down to the surface of the weather resistant steel or the surroundings appears. Flow rust not only impairs the landscape as a steel structure, but also causes environmental pollution. Therefore, various techniques for suppressing the occurrence of flow rust have been studied.

  For example, Japanese Patent Application Laid-Open No. 6-136557 proposes a surface treatment method in which a chromium sulfate aqueous solution or a copper sulfate aqueous solution is applied to a steel material and dried, and then further coated with an organic resin. Japanese Patent Application Laid-Open No. 8-13158 proposes a surface treatment method in which an aqueous solution containing aluminum is applied to a steel material and dried, and then coated with an organic resin. Furthermore, Japanese Patent Laid-Open No. 2000-212682 proposes a technique for suppressing flow rust by containing one or more of P, Cu, Ni, Cr, Mo, and B.

However, the techniques disclosed in Japanese Patent Laid-Open Nos. 6-136557 and 8-13158 consume a large amount of an aqueous solution and a resin, so that the surface treatment cost increases. In the technique disclosed in Japanese Patent Application Laid-Open No. 2000-212682, since a relatively expensive alloy element is used, the raw material cost of the steel material increases.
JP-A-6-136557 Japanese Patent Laid-Open No. 8-13158 JP 2000-212682 A

  The present invention provides a method for producing an inexpensive high corrosion resistant steel that can further improve the corrosion resistance of the weather resistant steel and suppress the occurrence of flow rust in view of the above-described problems of the prior art. With the goal.

  In order to further improve the corrosion resistance of the weathering steel and reduce the amount of flow rust, it is effective to select and add an alloy element to the weathering steel as appropriate. However, since the alloy elements added to the weathering steel are relatively expensive, increasing the amount of the alloy elements increases the raw material cost.

  Therefore, the present inventors have paid attention to the fact that the corrosion phenomenon is a chemical reaction between a metal phase of steel material (hereinafter referred to as “steel”) and oxygen or water. That is, in consideration of the progress of corrosion due to contact with oxygen or water on the surface of the base iron, the progress of the corrosion can be suppressed by concentrating the alloy elements near the surface of the base iron. In addition, since the alloy element is not concentrated in the entire base iron but only in the vicinity of the surface, the amount of the alloy element used can be reduced (that is, the raw material cost can be reduced).

  From such a point of view, the present inventors paid attention to Ni having an action of enhancing the corrosion resistance of steel as an alloy element to be concentrated near the surface of the ground iron, and intensively studied a technique for concentrating the Ni. In other words, in addition to Ni, P, Cu, Cr, Mo, and B are effective to suppress the occurrence of flow rust in a normal environment, but in order to increase the corrosion resistance in areas where salt comes in. Ni is the most effective.

  Therefore, the present inventors heated a steel slab containing 2.9% by mass of Ni (thickness: 210 mm) at 1120 ° C. for 2 hours and then rolled it into a steel plate (thickness 20 mm), and EPMA analysis of the cross section of the steel plate . A representative example of the result is shown in FIG. A region where Ni is concentrated (hereinafter referred to as a Ni-enriched layer) is observed near the surface of the base iron. The thickness of the Ni-enriched layer is about 5 μm, and the maximum Ni concentration in the Ni-enriched layer is about 5 times that of the ground iron.

  That is, when a steel material containing Ni is heated, Fe oxide called scale is generated on the surface, while Ni is concentrated near the surface of the ground iron. Here, the metal phase that constitutes the steel material is referred to as “steel” in distinction from the scale. At that time, Ni does not concentrate in the scale, but remains in the ground iron near the surface to form a Ni concentrated layer. Therefore, Ni added to the weathering steel is concentrated near the surface of the base iron to form a Ni concentrated layer, and the Ni concentrated layer remains even when used as a steel structure. The corrosion resistance of can be further increased.

  In addition, the vicinity of the surface of the ground iron means within 1000 μm from the surface of the ground iron. Even if it is far away from the surface, there is no effect in improving the corrosion resistance. The Ni-enriched layer refers to a layer that is at least 1.2 times the Ni content inside the base material.

  However, the conventional weathering steel uses a weathering steel that has been subjected to shot blasting in the production process and from which the scale of the surface has been removed. This is a treatment for forming a uniform protective rust without unevenness on the surface when used as a steel structure. In the shot blasting process, a steel ball having a diameter of about 1 mm is usually sprayed, so that the roughness of the surface of the steel after the scale is removed is about 60 μm in Rmax. Therefore, the Ni concentrated layer having a thickness of about 5 μm is removed together with the scale by the shot blasting process.

  When manufacturing the high corrosion resistance steel by applying the present invention, it is necessary to remove the scale on the surface while leaving the Ni concentrated layer in the vicinity of the surface of the base iron. However, it is inevitable that the Ni concentrated layer is removed together with the scale by mechanical means such as shot blasting or cutting. Therefore, only the scale is removed using chemical means. That is, it is possible to remove only the scale chemically by pickling and to leave the Ni concentrated layer in the vicinity of the surface of the base iron. Although it is not necessary to limit a pickling liquid to a specific component density | concentration, the hydrochloric acid aqueous solution generally used widely is preferable.

  Further, by adding an inhibitor to the pickling solution, it is possible to prevent elution of the base iron by pickling and leave the Ni concentrated layer.

  The present invention has been made based on the above findings.

That is, the present invention includes C: 0.01 to 0.20 mass%, Si: 0.05 to 0.80 mass%, Mn: 0.1 to 3.0 mass%, P: 0.005 to 0.1 mass%, S: 0.01 mass% or less, Al: 0.08 mass% or less. , Ni: Steel material having a composition containing 0.04 to 4.0% by mass with the balance consisting of Fe and inevitable impurities is heated to 900 to 1200 ° C. and then rolled to form a Ni concentrated layer having a thickness of 3 μm or more . Then, it is a manufacturing method of the high corrosion resistance steel which performs pickling and removes the scale of the surface of steel materials.
However, the Ni-enriched layer refers to a layer whose Ni content is 1.2 times or more of the Ni content inside the base material.

  In the manufacturing method of the high corrosion resistance steel of the present invention, the steel material is one or two of Cu: 0.1 to 1.0 mass%, Mo: 0.05 to 0.5 mass%, and Cr: 0.1 to 1.0 mass% in addition to the above-described composition. It is preferable to contain seeds or more. Furthermore, it is preferable that in addition to the above-described composition, the ground iron contains one or more of Nb: 0.005 to 0.1% by mass, Ti: 0.005 to 0.1% by mass, and V: 0.005 to 0.1% by mass.

  According to the present invention, it is possible to manufacture an inexpensive high corrosion resistant steel which can form a Ni concentrated layer in the vicinity of the surface of the ground iron to remarkably improve the corrosion resistance and suppress the occurrence of flow rust. Moreover, the high corrosion resistant steel manufactured by applying the present invention can maintain the excellent landscape of the steel structure by suppressing the occurrence of flow rust, so there is no need to apply a rust preventive paint, and the maintenance cost of the steel structure Can be reduced.

  First, the reason for limiting the components of the steel material to which the present invention is applied will be described.

C: 0.01-0.20% by mass
C is an element that increases the strength of the high corrosion-resistant steel. In order to obtain a desired strength, it is necessary to contain 0.01% by mass or more. On the other hand, if it exceeds 0.20% by mass, the toughness of the high corrosion resistant steel deteriorates. Therefore, C in the steel material needs to satisfy the range of 0.01 to 0.20 mass%.

Si: 0.05-0.80 mass%
Si is an element that acts as a deoxidizer in the melting stage of the molten steel and increases the strength of the high corrosion resistant steel. In order to obtain a desired strength, it is necessary to contain 0.05% by mass or more. On the other hand, if it exceeds 0.80% by mass, the toughness and weldability of the high corrosion resistance steel deteriorate. Accordingly, Si in the steel material needs to satisfy the range of 0.05 to 0.80 mass%.

Mn: 0.1-3.0 mass%
Mn is an element that increases the strength and toughness of the high corrosion-resistant steel. In order to obtain a desired strength, it is necessary to contain 0.1% by mass or more. On the other hand, if it exceeds 3.0% by mass, the toughness and weldability of the high corrosion resistance steel deteriorate. Therefore, Mn in the steel material needs to satisfy the range of 0.1 to 3.0% by mass.

P: 0.005 to 0.1% by mass
P is an element that densifies the protective rust and improves the corrosion resistance of the high corrosion resistant steel. If the P content is less than 0.005% by mass, the effect cannot be obtained. On the other hand, if it exceeds 0.1% by mass, the toughness of the high corrosion resistance steel deteriorates. Therefore, P in the steel material needs to satisfy the range of 0.005 to 0.1% by mass.

S: 0.01% by mass or less S is an impurity that is inevitably mixed in the melting stage of molten steel, and is an element that deteriorates the corrosion resistance, toughness, and weldability of high corrosion resistant steel. Therefore, S in steel materials shall be 0.01 mass% or less.

Al: 0.08 mass% or less
Al is an element that acts as a deoxidizer in the melting stage of molten steel, but not only deteriorates the toughness of high corrosion resistant steel, but also shifts to weld metal to deteriorate the toughness of welded joints. Therefore, Al in steel materials shall be 0.08 mass% or less.

Ni: 0.04-4.0 mass%
Ni is the most important element among the additive elements of the high corrosion resistance steel of the present invention. Ni increases the steel potential and improves the corrosion resistance of the high corrosion resistance steel. Moreover, by eluting it as ions in the protective rust, the rust grains are made finer and the formation of the protective rust is promoted. And it suppresses that a chloride ion permeate | transmits protective rust and reaches | attains a base iron. As a result, the generation of flow rust is suppressed at the initial stage when the steel structure is exposed to the atmosphere. Moreover, the corrosion resistance in the area where the salt content comes can be enhanced. This effect becomes more prominent as the Ni content increases. However, even if Ni is added excessively (ie, exceeding 4.0% by mass), the effect of suppressing flow rust cannot be expected, but rather the raw material cost increases due to an increase in Ni consumption.

  As already explained, when the weathering steel containing Ni is heated, a Ni concentrated layer is formed in the vicinity of the surface of the base iron. Ni does not concentrate in the scale, but in the iron bar near the surface. At this time, the Ni content of the Ni concentrated layer is about 5 times the Ni content of the base iron.

  According to the study by the present inventors, in order for the Ni concentrated layer to suppress the occurrence of flow rust, the Ni content of the Ni concentrated layer is preferably 0.2% by mass or more at the maximum value. Therefore, the Ni content of the ground iron needs to be 0.04 mass% or more. That is, Ni in the steel material needs to satisfy the range of 0.04 to 4.0 mass%.

  The thickness of the Ni concentrated layer needs to be 3 μm or more. The reason for this is that when the Ni concentrated layer is less than 3 μm, when the steel structure is constructed, wrinkles occur in the Ni concentrated layer and the steel is exposed, resulting in a decrease in corrosion resistance. On the other hand, even if the thickness of the Ni-concentrated layer exceeds 500 μm, an increase in the flow rust suppression effect cannot be expected, but rather an increase in manufacturing cost due to an increase in heat treatment time or the like is caused. Therefore, the thickness of the Ni concentrated layer is preferably 3 to 500 μm.

  In the steel material to which the present invention is applied, in addition to C, Si, Mn, P, S, and Ni, the following elements may be added as necessary.

Cu: 0.1 to 1.0 mass%
Cu forms protective rust early by making rust grains fine. This is an element having an action of suppressing the generation of flow rust. If the Cu content in the steel is less than 0.1% by mass, the effect cannot be obtained sufficiently. On the other hand, if it exceeds 1.0 mass%, not only the hot workability in the production process of high corrosion resistant steel is impaired, but also the increase in flow rust control effect cannot be expected, but rather the increase in raw material cost due to the increase in Cu consumption Invite. Therefore, when adding Cu in steel materials, the inside of the range of 0.1-1.0 mass% is preferable.

Mo: 0.05-0.5 mass%
Mo forms molybdate ions in the rust layer, thereby preventing chloride ions from passing through the rust layer and reaching the base iron. As a result, the generation of flow rust is suppressed at the initial stage when the steel structure is exposed to the atmosphere. In addition, the corrosion resistance in areas where salt comes in can be further enhanced. If the Mo content in the steel is less than 0.05% by mass, the effect cannot be obtained sufficiently. On the other hand, if it exceeds 0.5% by mass, not only the hot workability in the production process of high corrosion resistant steel is impaired, but also the increase effect of flow rust can not be expected, but rather the raw material cost increases due to the increase in Mo consumption Invite. Therefore, when adding Mo in steel materials, the range of 0.05-0.5 mass% is preferable.

Cr: 0.1 to 1.0 mass%
Cr forms protective rust early by making rust grains fine. This is an element having an action of suppressing the generation of flow rust. If the Cr content in the steel is less than 0.1% by mass, the effect cannot be obtained sufficiently. On the other hand, even if it exceeds 1.0 mass%, an increase in the flow rust suppression effect cannot be expected, but rather the raw material cost increases due to an increase in Cr consumption. Accordingly, when Cr is added to the steel material, it is preferably within the range of 0.1 to 1.0 mass%.

Nb: 0.005 to 0.1 mass%, Ti: 0.005 to 0.1 mass%, V: 0.005 to 0.1 mass%
Nb, Ti, and V are elements that increase the strength of the steel material, and one or more of them can be added as necessary. Nb, Ti, and V are all effective when contained in an amount of 0.005% by mass or more, but the effect is saturated even if each content exceeds 0.1% by mass. For this reason, it is preferable that all of Nb, Ti, and V be 0.005 to 0.1% by mass.

  Next, the manufacturing method of the high corrosion resistance steel of this invention is demonstrated.

  A molten steel having a predetermined composition is melted by a conventionally known technique such as a converter method or an electric furnace method, and then a steel material (for example, a steel slab) is manufactured by a continuous casting method or an ingot forming method. In addition, at the smelting stage of molten steel, refining technology (so-called primary refining) mainly using decarburization such as the converter method and electric furnace method, followed by refining technology mainly using degassing such as vacuum degassing method (so-called primary refining). Secondary refining) may be used in appropriate combination.

  Next, the steel material is heated to 900 to 1200 ° C. and further hot-rolled to obtain a steel material having a predetermined shape (that is, steel plate, shape steel, etc.), and then cooled by air cooling or accelerated cooling.

  Since the high corrosion resistant steel having a predetermined shape produced in this way has a scale formed on its surface, it is pickled to remove only the scale chemically. As the pickling solution, conventionally known ones such as dilute hydrochloric acid (ie, an aqueous solution of hydrochloric acid), dilute sulfuric acid (ie, an aqueous solution of sulfuric acid), dilute phosphoric acid (ie, an aqueous solution of phosphoric acid) can be used. However, it is preferable to use dilute hydrochloric acid in consideration of pickling efficiency and dissolution characteristics of the scale.

  Further, by adding an inhibitor to the pickling solution, it is possible to prevent elution of the base iron by pickling and leave the Ni concentrated layer. Inhibitors are appropriately selected and used according to the components of the base iron, the type of pickling solution, and the like.

  Molten steel having the components shown in Table 1 was melted using a converter, and a steel slab having a thickness of 210 mm was obtained by continuous casting. Steel numbers 1 to 11 in Table 1 all satisfy the component range of the steel material to which the present invention is applied. These steel materials (ie, steel slabs) were heated to 1120 ° C and then hot rolled to form steel plates with a thickness of 20 mm and a width of 2500 mm. The high corrosion-resistant steel plate thus obtained was pickled and the surface scale was removed. Dilute hydrochloric acid was used for the pickling solution. This is an invention example.

  On the other hand, as a comparative example, the high corrosion resistance steel plate manufactured in the same manner as the invention example was subjected to shot blasting to remove the surface scale.

For the inventive example and the comparative example, the Ni content (mass%) in the Ni concentrated layer, the thickness of the Ni concentrated layer (μm), and the amount of flow rust (μg / cm ² ) were investigated.

  When investigating the Ni content in the Ni-enriched layer and the thickness of the Ni-enriched layer, an analytical test piece (10 mm x 10 mm) was taken from the surface layer of each highly corrosion-resistant steel plate with the scale removed, and EPMA analysis was performed. Was done. Table 2 shows the maximum Ni content in the Ni concentrated layer. The maximum value of the Ni content in the Ni-concentrated layer was calculated from the Ni content (mass%) inside the base material × the maximum Ni detected strength of the Ni-concentrated layer × the average Ni detected strength inside the base material. In addition, Table 2 shows the thickness of the region in which the Ni content is 1.2 times or more of the inside of the base material (that is, the thickness of the Ni concentrated layer).

In investigating the amount of flow rust, corrosion test pieces (thickness 5 mm, width 50 mm, length 100 mm) including the steel plate surface were collected from each highly corrosion-resistant steel plate and subjected to a seawater spray test. The seawater spray test is a test in which seawater is sprayed twice a week on a corrosion test piece. A seawater spray test was conducted for one month, and the flow rust generated from the corrosion test pieces was placed in a plastic tank, and the mass of Fe ²⁺ was measured by atomic absorption spectrophotometry. Table 2 shows the values converted into values per 1 cm ^{2 of} corrosion test pieces.

  As is apparent from Table 2, in the inventive examples (steel symbols 1B to 11B), a Ni concentrated layer is formed in the vicinity of the surface of the high corrosion resistant steel plate. On the other hand, in the comparative example (steel symbols 1A to 11A), there is no Ni concentrated layer in the vicinity of the surface of the high corrosion resistant steel plate.

  Furthermore, regarding the amount of flow rust generated in each high corrosion resistance steel sheet, comparing the inventive example with the same steel number and the comparative example, the inventive example reduces the amount of flow rust even though it is a steel sheet of the same component. is doing.

  From this, it can be seen that the presence of the Ni concentrated layer exhibits a great effect in reducing the amount of flow rust generation.

It is a graph which shows concentration distribution of Fe, Ni, and O.

Claims (3)

C: 0.01 to 0.20 mass%, Si: 0.05 to 0.80 mass%, Mn: 0.1 to 3.0 mass%, P: 0.005 to 0.1 mass%, S: 0.01 mass% or less, Al: 0.08 mass% or less, Ni: 0.04 to A steel material containing 4.0% by mass with the balance consisting of Fe and inevitable impurities is heated to 900-1200 ° C. to form a Ni-enriched layer with a thickness of 3 μm or more , then rolled, and then pickled. A method for producing a highly corrosion-resistant steel, wherein the scale on the surface of the steel material is removed.
However, the Ni-enriched layer refers to a layer whose Ni content is 1.2 times or more of the Ni content inside the base material.   The steel material contains one or more of Cu: 0.1 to 1.0 mass%, Mo: 0.05 to 0.5 mass%, and Cr: 0.1 to 1.0 mass% in addition to the composition. Item 2. A method for producing a highly corrosion-resistant steel according to Item 1.   The steel material contains one or more of Nb: 0.005-0.1 mass%, Ti: 0.005-0.1 mass%, and V: 0.005-0.1 mass% in addition to the composition. Item 3. A method for producing a highly corrosion-resistant steel according to Item 1 or 2.

Images