KR101772812B1 - Steel material and method for producing the same - Google Patents

Abstract

A steel material capable of exhibiting excellent temporary antirust effect, and a method for producing the steel material.
Wherein the base steel is a steel material in which a proper amount of one or more of Ti, Nb and Zr is added in addition to Cu and Cr, and the scale layer contains Fe ₃ O ₄ : 40 To 80% of Fe ₂ O ₃ , and the remainder being other oxides containing Fe ₂ O ₃ , wherein the mass ratio of Fe ₃ O ₄ and Fe ₂ O ₃ in the scale layer is Fe ₂ O ₃ / Fe ₃ O ₄ And an average thickness of the scale layer is 3 to 15 mu m.

Description

TECHNICAL FIELD [0001] The present invention relates to a steel material and a method of manufacturing the steel material.

The present invention relates to a steel material used for a steel structure such as a ship, an offshore structure, a plant, a bridge, a building, etc., and a method for manufacturing the steel material. More particularly, the present invention relates to a steel material used in a corrosive environment And a method of manufacturing the steel material.

Steel is widely used as a structural member in many fields such as ships, offshore structures, plants, bridges, civil engineering, and construction, but it may cause corrosion or corrosion in the field of storage, drying process, or construction process. These blooming and corrosion have a cosmetic problem, and in the case of finally coating, adverse effects on coating quality are often caused.

In the case of corrosion or corrosion, for example, the mill scale or rust of the steel is removed by blast treatment as a pretreatment of painting. However, when the rust is remarkable, even if blast treatment is performed, rust or local corrosion remains in the steel, There is a case where coating defects are caused and the original anticorrosion performance of the coated film is not obtained.

The corrosion and corrosion of the steel in such storage, drying and construction processes are particularly troublesome in the case of construction under corrosive environments such as coastal areas and industrial areas, It is often the case that the steel material is exposed to the corrosive environment over a long period of time.

As a temporary rust prevention method for preventing the corrosion and corrosion of the steel in such a storage or drying and construction process, application of various paints such as zinc rich primer, surface treatment agent or various rust preventive oil is practically used.

For example, a temporary rust preventing method of applying a zinc rich primer having a dry film thickness of 10 to 20 占 퐉 on the surface of a steel material is effective in a relatively mild atmospheric corrosive environment in which there is little flying salinity, It is possible to prevent chewing. Moreover, the steel material coated with zinc rich primer is widely used because of its good weldability and good workability.

However, in a severe corrosive environment such as a large amount of saline, or when a period of time in which a steel material is left unused for a long period of time, the corrosion resistance is often insufficient. Although it is possible to improve the corrosion resistance of the steel material by thickening the film formed by applying the zinc rich primer, the steel material obtained by thickening the above film is welded to the weld bead by a blowhole or the like It is likely to cause defects, so that it is often difficult to thicken the coating formed by applying the zinc rich primer.

Further, when various anticorrosive paints and surface treatment agents are applied to secure the temporary antirust performance, the drying and curing properties of the coating agent may be lowered depending on the temperature and humidity at the time of application, and sufficient rustproofing ability may not be exhibited .

Further, in addition to lowering the handling properties of the steel material, the application of the anti-rust oil may cause welding defects, paint defects, and the like when the oil remains in the remaining state.

In recent years, for example, as disclosed in Patent Document 1, temporary anti-rust paints and the like have been proposed. Various anti-rust coating agents have been developed in accordance with each problem, but there are still many problems from a practical point of view.

On the other hand, on the surface of the steel, a scale such as a mill scale mainly composed of iron oxide and a scale such as black iron are produced in a manufacturing process such as hot rolling. Since the scale layer formed on the surface of the steel can be a protective coating in a corrosive environment when it is sound, it has an effect of suppressing the smell of the steel.

However, in the case of a normal steel material, the adhesion between the scale layer and the base steel is insufficient, so that the scale layer is cracked during each step or transportation after the hot rolling, or the scale layer is peeled off from the base steel, In many cases.

With respect to the scale of the steel, there is little attempt to positively utilize it as a means for temporary anticorruption. However, there is a high demand for improving the adhesion of the scale from the viewpoint of adverse effects on each step after hot rolling and poor appearance, A steel material, for example, is proposed in Patent Document 2.

However, the scale layer formed on the surface of the conventional base steel has a problem in that, even if it is in close contact with the base steel, if there is a minute crack or when it is not dense, a corrosive substance such as moisture or Cl ion It may invade and lead to corrosion due to corrosion of the base steel.

Even if the steel sheet is coated with the scale layer until the completion of the production of the steel material, if the steel sheet needs to be stored for a long time in the use environment from the time of shipment after shipment, the scale layer will peel off due to temperature changes, There are many cases.

For this reason, it is not necessarily sufficient to apply the technique of improving the adhesion between the base steel and the scale layer in the prior art in order to secure sufficient temporary anticorrosion performance for the scale layer on the surface of the steel.

Japanese Patent Application Laid-Open No. 2013-43986 Japanese Patent Application Laid-Open No. 2014-4610

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above-described conventional circumstances, and its main object is to provide a steel material capable of exhibiting excellent temporary antirust effect and a method of manufacturing the steel material. Further, other objects of the present invention will become apparent from the description of the present specification.

The steel material according to the present invention is a steel material comprising a base steel and a scale layer formed on the surface of the base steel, wherein the base steel comprises, by mass%, 0.04 to 0.3% of C, 0.1 to 1.0% 0.1 to 2.5% of Mn, more than 0 to 0.04% of P, and 0.04 to less than 0.04% of S, 0.01 to 0.2% of Al, 0.05 to 1.0% of Cu, 0.05 to 1.0% of Cr, 0.005 to 0.1% of Ti, 0.005 to 0.1% of Nb and 0.005 to 0.1% of Zr, and the balance of Fe and inevitable impurities , And the scale layer contains 40 to 80% of Fe ₃ O _{4 in} terms of% by mass with respect to the entire scale layer, the balance being other oxides containing Fe ₂ O ₃ , The mass ratio of Fe ₃ O ₄ and Fe ₂ O ₃ to Fe ₂ O ₃ / Fe ₃ O ₄ is 0.2 to 0.4, and the average thickness of the scale layer is 3 to 15 μm.

Further, it is preferable that the base steel further contains at least one of Ni, 0.05 to 6.0%, Co: 0.01 to 5.0%, Mo: 0.01 to 2.0% and W: 0.01 to 2.0% .

Further, it is preferable that the above-mentioned ground steel further contains at least one or more of 0.0005 to 0.01% of Mg, 0.0005 to 0.01% of Ca and 0.0005 to 0.01% of REM in terms of mass%.

It is also preferable that the base steel further contains at least one of Sn, 0.001 to 0.2%, Sb: 0.001 to 0.2% and Se: 0.001 to 0.2% in mass%.

Further, it is preferable that the above-mentioned base steel further contains at least one of B, 0 to 0.01%, V: more than 0% to 0.1%, and Zn: 0 to 0.1% .

In the method of manufacturing a steel material according to the present invention, the steel ingot is heated to 1000 to 1200 占 폚 and then subjected to hot rolling to produce a steel material, wherein the rolling finish temperature of the hot rolling is 660 And the cooling rate after hot rolling is set to 1 to 10 占 폚 / s, and the cooling rate of the hot rolling is set to 1 to 10 占 폚 / s .

Effects obtained by the invention disclosed in this specification will be briefly described as follows. That is, according to the present invention, it is possible to realize a steel material that exhibits excellent temporary antirust effect.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory view showing a thickness measurement point of a scale layer of a test piece used in the embodiment. FIG.

Generally, the scale layer generated on the surface of the base steel is peeled off during storage at the site after production of the steel or during construction, and as a result, the exposed base steel is corroded to lead to brittle. Further, even in a portion where the scale layer is not peeled off because of good adhesion to the base steel, corrosive substances such as water and Cl ions penetrate into the inside through a defective portion such as a minute crack or an unclean portion of the scale layer, , It causes deep corrosion in the base steel, which is often a problem in the post-processing.

The inventors of the present invention have studied the use of a scale layer generated at the time of manufacturing a steel material as a means of temporary rust prevention in order to solve the problem of deterioration of weld quality and paint quality deterioration caused by rust preventive coating agents such as various paints, surface treatment agents or anti- . Specifically, in addition to the improvement of the adhesion between the scale layer and the base steel, the improvement of the protection by the densification of the scale layer itself and the inhibition of corrosion of the steel itself in the scale-defective portion were examined.

As a result, it has been found that by optimizing the scale production conditions in the production process of the base steel material in which a proper amount of one or more of Ti, Nb or Zr is added in addition to Cu and Cr, The scale layer can be formed on the surface of the non-coated steel, and a good temporary antirust effect can be obtained over a long period of time. Further, it has been found that the corrosion of the base steel in the scale-defective portion inevitably can be suppressed by appropriately adjusting the content of the additive elements such as Cu and Cr in the steel.

As described above, according to the present invention, the generation of rust is delayed by the improvement of the protective property of the scale layer itself, and furthermore, the excellent temporary rust preventive performance is obtained by suppressing the progress of corrosion of the base steel itself in the scale defective portion. Further, in the present invention, it is necessary to optimize the composition and thickness of the scale layer in addition to the optimization of the component composition of the base steel, and the reasons for limitation thereof are described below.

<Composition of Substrates>

In addition to excellent corrosion resistance, the under-floor steel needs to satisfy various characteristics such as mechanical properties and weldability required as structural members. From these viewpoints, it is necessary to appropriately adjust the contents of Si, Mn, Al, P, and S in addition to the above-mentioned elements such as Cu, Cr, Ti, Nb and Zr that improve temporal rust resistance and corrosion resistance of the base steel sheet. Hereinafter, the reason for limiting the range of components of these various added elements will be described. Hereinafter, all the units including the description of the table are expressed in%, and all represent% by mass.

C: 0.04 to 0.3%

C is a basic additive element necessary for securing the strength of the base steel. In order to obtain the strength characteristic generally required as the base steel, it is necessary to contain at least 0.04% or more. However, when C is excessively contained, in addition to deterioration of toughness, cementite acting as a cathode site in the peeling portion of the scale layer accelerates the corrosion reaction and deteriorates the corrosion resistance. In order not to cause such an adverse effect by excessive addition of C, it is necessary to suppress the content of C to 0.3% at most. Therefore, the content of C was 0.04 to 0.3%. On the other hand, the lower limit of the content of C is preferably 0.045%, more preferably 0.05% or more. The upper limit of the content of C is preferably 0.28%, more preferably 0.25% or less.

Si: 0.1 to 1.0%

Si is an element that is effective for improving the adhesion between the base steel sheet and the scale layer, and is also an element necessary for deoxidization and strength. In order to obtain these effects, it is necessary to contain at least 0.1% or more. However, when Si is contained in excess of 1.0%, the weldability is deteriorated. Therefore, the content of Si was set to 0.1 to 1.0%. On the other hand, the lower limit of the Si content is preferably 0.11%, more preferably 0.12% or more. The upper limit of the Si content is preferably 0.95%, more preferably 0.90% or less.

Mn: 0.1 to 2.5%

Like Mn, Mn is an element necessary for deoxidation and strength assurance. If it is less than 0.1%, it is impossible to secure the lowest strength as a base steel used as a structural member. However, if it is contained in excess of 2.5%, the toughness deteriorates. Therefore, the content of Mn was set to 0.1 to 2.5%. On the other hand, the lower limit of the Mn content is preferably 0.15%, more preferably 0.2% or more. The upper limit of the Mn content is preferably 2.4%, more preferably 2.3% or less.

P: not less than 0% and not more than 0.04%

P is an element that deteriorates toughness and weldability when contained in excess, and the upper limit of the allowable content of P is 0.04%. The content of P is preferably as small as possible, and a more preferable upper limit of the content of P is 0.038%, more preferably 0.035% or less. However, it is difficult to industrially make P in the base steel to 0%.

· S: more than 0% and less than 0.04%

When the S content is increased, the element tends to deteriorate toughness and weldability. The upper limit of the allowable content is 0.04%. A more preferable upper limit of the content of S is 0.038%, more preferably 0.035% or less. However, it is difficult to industrially make S in the base steel to 0%.

Al: 0.01 to 0.2%

Al is an element necessary for deoxidation and strength assurance like Si and Mn described above. In order to exhibit such action effectively, it is necessary to contain 0.01% or more. However, if it is contained in an amount exceeding 0.2%, the weldability is deteriorated. Therefore, the content of Al is in the range of 0.01 to 0.2%. On the other hand, the lower limit of the content of Al is preferably 0.011%, more preferably 0.012% or more. The upper limit of the content of Al is preferably 0.19%, more preferably 0.18% or less.

Cu: 0.05 to 1.0%

Cu in the steel is an additive element that acts to enhance the durability by densifying the scale layer. Furthermore, Cu is an element having an action to suppress the expansion of corrosion from scale scratches and the like because it has an action of reducing the activity of the anode of the base steel material exposed in the defective portion or peeling portion of the scale layer and densifying the rust. In order to exhibit such an effect, it is necessary to contain at least 0.05% or more. However, if it is contained in an excess amount, the weldability and hot workability deteriorate, so the content of Cu should be 1.0% or less. Therefore, the content of Cu is set to 0.05 to 1.0%. On the other hand, the lower limit of the Cu content is preferably 0.06%, and more preferably, the lower limit is 0.07%. The preferable upper limit of the Cu content is 0.95%, and the more preferable upper limit is 0.9%.

Cr: 0.05 to 1.0%

Cr, like Cu, is an additive element that acts to enhance durability by densifying the scale layer. In addition, Cr is an element having an action to suppress the expansion of corrosion from scale scratches and the like because it has an action of reducing the activity of the anode of the base steel material exposed in the defective portion or peeling portion of the scale layer and densifying the rust. In order to exhibit such an effect, it is necessary to contain at least 0.05% or more. However, if it is contained in an excess amount, the weldability and hot workability deteriorate. Therefore, the content of Cr should be 1.0% or less. Therefore, the content of Cr was set in the range of 0.05 to 1.0%. On the other hand, the lower limit of the Cr content is preferably 0.06%, and the lower limit is more preferably 0.07%. The preferable upper limit of the Cr content is 0.95%, and the more preferable upper limit is 0.9%.

Ti: 0.005 to 0.1%, Nb: 0.005 to 0.1%, and Zr: 0.005 to 0.1%

Ti, Nb and Zr have the function of densifying the rust of the base steel material in the scratched portion in the coexistence of Cu and Cr, and are elements necessary for suppressing corrosion progress from scale scratches. In order to exhibit such an effect, it is necessary to contain at least 0.005% each. However, if it is contained in an excessive amount, the weldability and hot workability deteriorate. Therefore, the contents of Ti, Nb and Zr must be 0.05% or less, respectively. The preferable lower limit of the content of Ti, Nb and Zr is 0.006%, and the lower limit is more preferably 0.007%. The preferable upper limit of the content of Ti, Nb and Zr is 0.095%, and the upper limit is more preferably 0.09%.

N: 0.001 to 0.015%

N is an additive element having an action of improving durability by densifying the scale layer in coexistence with any of Ti, Nb and Zr. In order to obtain such an effect, the N content needs to be 0.001% or more. However, if the content is excessive, the toughness of the base steel is adversely affected, and the weldability is deteriorated. Therefore, the upper limit of the content of N is set to 0.015%. Therefore, the range of the content of N was set to 0.001 to 0.015%. On the other hand, the lower limit of the content of N is preferably 0.0015%, more preferably 0.002% or more. The preferable upper limit of the content of N is 0.014%, more preferably 0.013% or less.

The above is the reason for limiting the range of the essential additive elements of the base steel constituting the steel material of the present invention, and the balance is Fe and inevitable impurities. The inevitable impurities include O and H, and these elements may be contained to such an extent as not to impair various properties of the base steel. However, by suppressing the total content of these inevitable impurities to 0.1% or less, preferably 0.09% or less, the effect of manifesting the corrosion resistance according to the present invention can be maximized.

Further, it is more effective to contain the following elements in the base steel constituting the steel material of the present invention. The reason for limiting the range of components when these elements are contained will be described below.

One or more of Ni: 0.05 to 6.0%, Co: 0.01 to 5.0%, Mo: 0.01 to 2.0%, and W: 0.01 to 2.0%

Ni, Co, Mo, and W have an effect of lowering the activity of the corrosion reaction of the base steel in the defective portion of the scale layer, and are effective elements for improving the corrosion resistance. The appropriate amounts of Ni, Co, Mo and W are also effective for improving the strength characteristics of the base steel and are added as required. In order to exhibit such an effect, it is preferable to contain 0.05% or more of Ni and 0.01% or more of Co, Mo, and W, respectively. However, excessive amounts of these elements deteriorate the weldability and hot workability. When they are contained, the content of Ni is 6.0% or less, that of Co is 5.0% or less, and that of Mo and W is 2.0% or less. A more preferable lower limit when Ni is contained is 0.06%, more preferably 0.07% or more. A more preferable lower limit when Co, Mo and W are contained is 0.02%, more preferably 0.03% or more. A more preferable upper limit when Ni is contained is 5.9%, more preferably 5.8% or less. A more preferable upper limit when Co is contained is 4.9%, more preferably 4.8% or less. A more preferable upper limit when Mo and W are contained is 1.9%, more preferably 1.8% or less.

Mg: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, and REM: 0.0005 to 0.01%

Mg, Ca, and REM have an effect of restricting the pH decrease in the vicinity of the surface of the base steel in the defective portion of the scale layer, and are effective elements for further improving the corrosion resistance. This action is exerted when these elements are dissolved by corrosion and react with hydrogen ions. In order to exhibit such an effect effectively, it is preferable that each is contained in an amount of 0.0005% or more. However, when the content of these elements is excessive, the weldability and hot workability deteriorate. When these elements are contained, the content is made 0.0005 to 0.01%. When Mg, Ca and REM are contained, the lower limit is more preferably 0.0006%, and still more preferably 0.0007%. On the other hand, when Mg, Ca and REM are contained, the more preferable upper limit is 0.0095%, and the more preferable upper limit is 0.009%, respectively.

0.001 to 0.2% of Sn, 0.001 to 0.2% of Sb, and 0.001 to 0.2% of Se.

Sn, Sb and Se are effective addition elements for suppressing corrosion of defects in the scale layer. This effect is effectively exhibited by containing 0.001% or more of each of these elements. However, when the content of these elements is excessive, the weldability and hot workability deteriorate. When these elements are contained, the content is made 0.001 to 0.2%. A more preferable lower limit when Sn, Sb and Se are contained is 0.002%, and a more preferable lower limit is 0.003%, respectively. On the other hand, when containing Sn, Sb and Se, the more preferable upper limit is 0.19%, and the more preferable upper limit is 0.18%, respectively.

B: not less than 0% but not more than 0.01%, V: not less than 0% and not more than 0.1%, Zn: not less than 0% and not more than 0.1%

B which is effective for improving the strength of V, etc., and Zn which is effective for improving toughness can be contained as needed. When these added elements are contained in an extremely small amount, the effect of improving the strength or toughness is exhibited. For example, when the content of B is 0.0001% or more and the content of V and Zn is 0.001% or more, the strength or toughness improving effect is more reliably expressed. However, if B and V are included excessively, the toughness of the base material deteriorates. If Zn is contained excessively, the weldability deteriorates. Therefore, the content of these elements is limited. When B is contained, it is not more than 0.01%. When V and Zn are contained, each is not more than 0.1%. The more preferable lower limit when B is contained is 0.0002%, and the more preferable lower limit is 0.0003%. On the other hand, when B is contained, the more preferable upper limit is 0.0095%, and the more preferable upper limit is 0.009%. Further, when V and Zn are contained, a more preferable lower limit is 0.002%, and a more preferable lower limit is 0.003%. On the other hand, a more preferable upper limit when V and Zn are contained is 0.095%, and a more preferable upper limit is 0.09%.

<Scale layer>

In a conventional steel material, a scale layer made of Fe ₂ O ₃ , Fe ₃ O ₄ , FeO, (Fe, Mn) O, Fe ₂ SiO ₄ , an oxide of other alloying elements, etc., Is formed on the surface. Of these factors, the factor greatly influencing the temporary anticorrosiveness of the scale is the amount of Fe ₂ O ₃ and Fe ₃ O _{4 in} the scale layer in addition to the adhesion of the scale layer.

In the present invention, the adhesion of the scale layer and the compactness of the scale layer are improved by the action of the chemical component of the base steel, so that the penetration of corrosive factors such as Cl ions into the base steel is suppressed to enhance the anti-

In order to obtain the excellent rust inhibitive performance of the scale layer over a long period of time, in addition to the optimization of the composition of the above-mentioned base steels, the content ratio of Fe ₃ O _{4 in} the scale layer, the mass ratio of Fe ₂ O ₃ and Fe ₃ O _{4 in} the scale layer, And the thickness of the scale layer.

The mass ratio of a present invention the ratio content of of the scale layer Fe ₃ O ₄ is to the mass% of Fe ₃ O ₄ in the entire scale layer is formed on the steel material surface according to, Fe ₂ O ₃ and Fe ₃ O ₄ is the steel material surface The mass ratio of Fe ₂ O ₃ and Fe ₃ O ₄ in the scale layer formed on the substrate, that is, Fe ₂ O ₃ / Fe ₃ O ₄ .

These can be obtained by quantitative analysis of a steel material having a scale layer formed thereon by an X-ray diffraction method. Concretely, X-rays monochromated from the surface of the steel material on which the scale layer is formed are irradiated to obtain respective concentrations from the areas of peaks corresponding to Fe ₂ O ₃ and Fe ₃ O ₄ of diffracted X-rays, By subtracting the concentration of Fe from the entire scale layer.

Hereinafter, the content ratio of Fe ₃ O _{4 in} the scale layer, the mass ratio of Fe ₂ O ₃ and Fe ₃ O _{4 in} the scale layer, and the thickness of the scale layer will be described in detail.

The content of Fe ₃ O _{4 in} the scale layer

Fe ₃ O ₄ in the scale layer has an effect of improving the adhesion between the scale layer and the base steel sheet, and is necessary to maintain the anticorrosion property for a long time in the use environment. In order to obtain such an effect, it is necessary that the ratio of Fe ₃ O ₄ occupied in the scale layer is 40% or more by mass%. However, if the ratio of Fe ₃ O ₄ occupied in the scale layer becomes excessive, corrosion of the defect portion of the scale layer is promoted, so that the ratio of Fe ₃ O ₄ occupied in the scale layer needs to be 80% or less by mass%. On the other hand, the proportion of Fe ₃ O _{4 in} the scale layer is more preferably 41% or more, and more preferably 42% or more. The proportion of Fe ₃ O _{4 in} the scale layer is more preferably 79% or less, and more preferably 78% or less.

The mass ratio of Fe ₂ O ₃ and Fe ₃ O _{4 in} the scale layer: Fe ₂ O ₃ / Fe ₃ O ₄

Fe ₂ O ₃ is generated on the outermost surface of the scale layer and is necessary to improve the protection of the scale layer. If the proportion of Fe ₂ O ₃ occupying in the scale layer is excessively large, peeling of the scale layer tends to occur easily. On the other hand, if the proportion is too small, the protective property improving action is not obtained. The optimum Fe ₂ O ₃ content in the scale layer can not be determined by Fe ₂ O ₃ alone, but is determined by the mass ratio to Fe ₃ O ₄ in the scale layer. Specifically, the mass ratio determined by Fe ₂ O ₃ / Fe ₃ O ₄ should be 0.2 to 0.4. On the other hand, the lower limit of the mass ratio of Fe ₂ O ₃ and Fe ₃ O _{4 in} the scale layer: Fe ₂ O ₃ / Fe ₃ O ₄ is more preferably 0.21, and still more preferably 0.22 or more. On the other hand, the upper limit of Fe ₂ O ₃ / Fe ₃ O ₄ is more preferably 0.39, and still more preferably 0.38 or less.

As described above, the scale layer in the steel material of the present invention is composed of an oxide mainly composed of Fe ₂ O ₃ and Fe ₃ O ₄ . Examples of the other oxides include oxides containing at least one of iron and an alloy element, and examples thereof include FeO, (Fe, Mn) O, Fe ₂ SiO ₄ , and the like.

· Thickness of scale layer

The thickness of the scale layer needs to be optimized in order to maintain the anticorrosion in the use environment for a long time. If the scale layer is too thin, the effect of inhibiting the penetration of corrosive factors such as Cl ions into the base steel becomes insufficient, and sufficient pantoscity is not obtained. On the other hand, if the scale layer is too thick, scale separation due to temperature changes in the night and day in the use environment tends to occur, and the anticorrosion can not be maintained for a long time. From this point of view, it is necessary to set the thickness of the scale layer to an average thickness of 3 占 퐉 to 15 占 퐉. A more preferable lower limit of the average thickness of the scale layer is 3.5 mu m, and a more preferable lower limit is 4 mu m. Further, a more preferable upper limit of the average thickness of the scale layer is 14.5 mu m, and a more preferable upper limit is 14 mu m.

The thickness of the scale layer can be measured from the cross-sectional observation of the steel material, or it can be measured using an ultrasonic thickness meter. On the other hand, since the thickness of the scale layer may differ depending on the measurement position of the steel sheet, it is preferable that the mean value of arbitrary ten points or more, that is, the average thickness is the thickness of the scale layer.

<Manufacturing Method of Steel>

In order to reliably produce the base steel, for example, it can be manufactured by the method described below. First, the molten steel introduced from a converter or an electric furnace into a ladle is adjusted to the composition specified in the present invention by using an RH vacuum degassing apparatus, and the temperature is adjusted to 2 Car refining is performed. Thereafter, it is formed into a steel ingot by a conventional casting method such as a continuous casting method and a roughing method. On the other hand, as a structural member, in order to secure basic characteristics such as mechanical properties and weldability required for a steel material, it is preferable to use killed steel as a deoxidation type, and more preferably to use an Al killed steel.

The obtained ingot is heated to 1000 to 1200 占 폚 and then subjected to hot rolling. When the reduction rate is large, it is possible to perform two-stage rolling such as rough rolling and finish rolling. At this time, in order to prevent the oxide formed by the heating before rolling from digging into the base steel to prevent the damage of the scale, which is finally produced, it is recommended to thoroughly remove the scale by descaling before the rolling.

On the other hand, in the steel material in which the scale layer of the present invention is formed on the surface, for example, the desired scale composition and thickness can be obtained by adjusting the rolling finish temperature, the time from the end of rolling to the start of cooling, and the atmosphere.

If the rolling finishing temperature is too low, the amount of Fe ₃ O _{4 in} the scale becomes small, and a predetermined scale composition can not be obtained. In addition, when the rolling finishing temperature is too high, in addition to the formation of a thick scale layer having many pores, the FeO amount is increased, so that a predetermined scale composition can not be obtained. For this reason, the rolling finishing temperature can be set in the range of 660 캜 to 770 캜. A more preferable temperature of the rolling finish temperature is 680 占 폚 for the lower limit and 750 占 폚 for the upper limit.

Here, at the time of steel product production, it is a general method to start cooling without time from the end of the hot rolling until the start of cooling, and usually the cooling is started within approximately 20 seconds to 60 seconds. On the other hand, in the present invention, in order to form a scale layer having a predetermined composition and thickness, it is necessary to secure some time from the end of hot rolling to the start of cooling.

If the time from the end of rolling to the start of cooling is too short, the amount of Fe ₃ O ₄ becomes too small to achieve a predetermined scale composition, and since the scale does not grow sufficiently, a scale layer of a predetermined thickness is not obtained . On the other hand, if the time from the end of rolling to the start of cooling is too long, the Fe ₂ O ₃ / Fe ₃ O ₄ ratio becomes excessively high and a predetermined scale composition can not be obtained. In addition, the scale layer becomes too thick, And it becomes easy to cause scale peeling in a cooling process or the like at a later time. From this point of view, the time from the end of rolling to the start of cooling is 30 to 120 seconds.

On the other hand, during the period from the end of rolling to the start of cooling, it is recommended to keep the temperature within the range of -50 캜 to the rolling finish temperature in order to grow a predetermined scale.

Further, in the case of ordinary hot rolling, in many cases, an atmosphere having a relatively large amount of water vapor is generated for water cooling in a later cooling process, and it is generally rolled in an environment of relatively high humidity. However, in the property of the scale layer, moisture in the atmosphere greatly influences the moisture content. When the moisture content is excessively high, the scale densities are lowered, and the effect of inhibiting intrusion of corrosive substances such as Cl ions is not obtained.

Therefore, in the present invention, it is also necessary to control the humidity of the atmosphere at the time of rolling. For this reason, it is necessary to set the moisture content in the atmosphere exposed from the descaling before hot rolling to the start of cooling after hot rolling to 70% RH or less at relative humidity. On the other hand, the moisture content in the atmosphere is more preferably 60% RH or less, and more preferably 50% RH or less.

In addition, since the cooling process after hot rolling affects the growth and properties of the scale, it is also necessary to adjust the cooling rate. If the cooling rate after the hot rolling is too slow, the scale layer becomes excessively thick due to the scale growth at the time of cooling, and a scale layer having a predetermined thickness can not be obtained. If the cooling rate after the hot rolling is too fast, heat cracks are generated in the scale layer and the protective property of the scale layer is deteriorated. From this point of view, it is recommended that the cooling rate after hot rolling is set to 1 to 10 DEG C / s. On the other hand, a more preferable lower limit of the cooling rate is 1.5 占 폚 / s, and a more preferable lower limit is 2 占 폚 / s. On the other hand, a more preferable upper limit of the cooling rate is 9 占 폚 / s, and a more preferable upper limit is 8 占 폚 / s.

On the other hand, examples of the form of the steel material of the present invention include steel sheet, steel pipe, bar steel, wire rod, section steel and the like. Examples of applications include those used as structural members of ships such as cargo ships such as tankers, container ships, and bulker, and vans, passenger ships, and military ships. In addition, the offshore structures include structures for excavating oil and natural gas in the ocean, floating structures for producing, storing and extracting oil and gas from the ocean, wind power generation in the ocean, , Structural members for use in power generation facilities such as algae and ocean current generation, temperature difference generation, and photovoltaic power generation.

Example

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the present invention is not limited to the following Examples, of course, All of which are included in the technical scope of the present invention.

[Production of the disclosure material]

A steel material having various compositional compositions shown in Tables 1 and 2 was dissolved in a vacuum melting furnace to obtain a steel ingot of 50 kg. The obtained ingot was heated to 1100 캜, descaled by high-pressure water jet, and then hot-rolled to obtain a steel material having a thickness of 10 mm.

At this time, as shown in Table 3, 1 to 7 and 14 to 52, the end temperature of hot rolling is appropriately adjusted in the range of 660 to 770 캜, the time from the end of hot rolling to the start of cooling is appropriately adjusted within a range of 30 to 120 seconds, The humidity of the atmosphere in which the steel sheet is exposed is appropriately adjusted to not more than 70% RH and the cooling rate after hot rolling is appropriately adjusted in the range of 1 to 10 ° C / s, Were adjusted so as to satisfy the following conditions.

In addition, In Examples 8 to 13, as shown in Table 3, the end temperatures of the hot rolling, the time from the end of hot rolling to the start of cooling, the humidity of the atmosphere, and the cooling rate were changed so that the chemical composition satisfied the requirements of the present invention, Was adjusted to deviate from the conditions of the present invention.

Three pieces of test specimens for testing atmospheric exposure, each having a size of 150 mm x 150 mm x 10 mm, were cut out from the obtained steel material, leaving the surface on which the scale layer was formed as it is. Further, in order to obtain the composition of the scale layer, a sample having a size of 40 mm x 40 mm x 10 mm was sampled from the steel material after hot-rolling, and quantitative analysis was performed by an X-ray diffraction method using a Co target. Scale thickness was measured using an ultrasonic thickness gauge with respect to 16 points of the test piece shown in Fig. 1, and the average value thereof was taken as an average thickness of the scale layer.

All test specimens were covered with a silicone sealant at a surface other than the one surface of 150 x 150 mm to be the test surface, and were disclosed in the following atmospheric exposure test.

[Waiting exposure test method]

In order to evaluate the temporary anti-rusting ability of each scale-attached steel material in an environment with severe corrosion due to scorching salinity, an atmospheric exposure test was conducted. The exposure site was located at a distance of 100 m from the coast of Kagawa Prefecture, Hyogo Prefecture, Japan, and the test piece of Fig. 1 was exposed so that the test surface of each test piece was at 45 ° C. Exposed test specimens are three each for each steel. The test specimens were sprayed with seawater once a week to promote corrosion. The exposure period is 56 days. As the temporary anticorrosion ability, the surface area and maximum corrosion depth of the test surface after exposure were evaluated.

As for the tearing area, the test surface after exposure was photographed with a digital camera, and the area of the tearing area was determined by image analysis, and the average value of the three test pieces was taken as the tearing area. For the maximum corrosion depth, the exposed test pieces were immersed in a 10% aqueous solution of ammonium hydrogencarbonate at a temperature of 80 DEG C and subjected to a de-caking treatment, and then the depth of corrosion of local corrosion sites was measured using a depth gauge. The deepest of these was the maximum corrosion depth.

The evaluation criteria are as follows. First, regarding the tearing area, A steel having a relative value of 50 or more and less than 60 was rated AA, and a steel having a relative value of less than 50 was rated AAA.

Further, for the maximum corrosion depth, 1, a relative value of not less than 60 and not more than 70 is AA, and a relative value of less than 60 is AAA.

It was evaluated that it was a steel material with excellent anti-rusting ability, and that both the tearing area and the maximum corrosion depth were A to AAA and that both items were A to AAA.

[Evaluation results]

Table 4 shows the evaluation results by the test. No. 1 to No. 13, the relative value of the tearing area and the maximum corrosion depth is 90 to 100, and the anti-rust property is not sufficient. No. 2 to No. 7 do not satisfy the conditions of the present invention, and the addition amounts of Cu, Cr, N, Ti, Nb and Zr are too small, so that the temporary anti-rust property is not sufficiently obtained.

No. 8 to No. The composition of the 13-element non-woven steel satisfies the conditions of the present invention, but the composition or thickness of the scale layer does not satisfy the conditions of the present invention. No. 8 indicates that the content ratio of Fe ₃ O _{4 in} the scale layer is larger than that of Fe ₃ O _{4 in} the scale layer. 9, the ratio of Fe ₂ O ₃ / Fe ₃ O _{4 is} excessively small, respectively, so that the temporary anti-rust property is not sufficiently obtained. No. 10, since the thickness of the scale layer is excessively thin, the temporary anti-rust property can not be sufficiently obtained.

In addition, 11 indicates that the content ratio of Fe ₃ O _{4 in} the scale layer is larger than that of Fe ₃ O _{4 in} the scale layer. 12, the ratio of Fe ₂ O ₃ / Fe ₃ O _{4 is} excessively large, so that the antifouling performance is not sufficiently obtained. No. 13, since the thickness of the scale layer is excessively thick, the temporary anti-rust property can not be sufficiently obtained.

Compared to these comparative examples, 14 ~ No. 52 are all suppressed to be less than 70 and the relative value of the maximum corrosion depth is suppressed to less than 80. [ As described above, in the case of satisfying all the conditions of the present invention. 14 ~ No. 52 steel exhibits excellent corrosion resistance in a corrosive environment of salinity salinity.

Particularly, it is possible to obtain a steel sheet having a scale layer satisfying the conditions of the present invention on the surface of S16 to S22 base steels containing at least one of Ni, Co, Mo and W in an appropriate amount. 22 to No. 28 is remarkably effective in suppressing the tingling area.

In each of these, a scale layer satisfying the conditions of the present invention was formed on the surface of S37 to S40 base steels containing any one or more of Sn, Sb and Se in an appropriate amount. 43 to No. 46 is more effective in suppressing the chewing area.

Further, a surface layer of S23 to S28 containing a proper amount of at least one of Mg, Ca and REM in an appropriate amount was coated on the surface of a non-coated steel sheet with a scale layer satisfying the conditions of the present invention. 29 to No. 34 is particularly remarkable for the effect of suppressing the maximum corrosion depth.

In each of these samples, a scale layer satisfying the conditions of the present invention was formed on the surface of S41 to S43 non-coated steels containing any one or more of Sn, Sb and Se in an appropriate amount. 47 to No. 49 is more effective in suppressing the maximum corrosion depth.

As described above, the steel material satisfying the conditions of the present invention, in which the scale layer is formed on the surface of the base steel material, exhibits excellent temporal rustproofing property under a severe atmospheric corrosive environment and can be used for ships, marine structures, It is suitable as a steel material to be used for steel structures such as field.

Claims (3)

A steel material comprising a base steel material and a scale layer formed on the surface of the base steel material,
Wherein the base steel comprises, by mass%, 0.04 to 0.3% of C, 0.1 to 1.0% of Si, 0.1 to 2.5% of Mn, P of more than 0 to 0.04%, S of more than 0 to 0.04% 0.005 to 0.1% of Ti, 0.005 to 0.1% of Nb, 0.005 to 0.1% of Zr, 0.01 to 0.2% of Cu, 0.05 to 1.0% of Cu, 0.05 to 1.0% of Cr and 0.001 to 0.015% of N, %, And the balance of Fe and inevitable impurities,
Wherein the scale layer contains, as a percentage by mass with respect to the entire scale layer, Fe ₃ O ₄ : 40 to 80%, and the remainder being Fe ₂ O ₃ -containing oxides,
The mass ratio of Fe ₃ O ₄ and Fe ₂ O ₃ in the scale layer is 0.2 to 0.4 in terms of Fe ₂ O ₃ / Fe ₃ O ₄ ,
Further, the average thickness of the scale layer is 3 to 15 占 퐉. The method according to claim 1,
A steel material comprising at least one of the following groups (a) to (d):
(a) at least one selected from the group consisting of Ni: 0.05 to 6.0%, Co: 0.01 to 5.0%, Mo: 0.01 to 2.0%, and W: 0.01 to 2.0%
(b) at least one selected from the group consisting of 0.0005 to 0.01% of Mg, 0.0005 to 0.01% of Ca, and 0.0005 to 0.01% of REM,
(c) 0.001 to 0.2% of Sn, 0.001 to 0.2% of Sb, and 0.001 to 0.2% of Se,
(d) at least one selected from the group consisting of B: more than 0% to 0.01% or less, V: more than 0% to 0.1% or less, and Zn: more than 0% to 0.1% A method for producing a steel material by heating a steel ingot having the component composition according to claim 1 or 2 to 1000 to 1200 캜 and hot rolling the steel ingot,
The rolling finish temperature of the hot rolling is set to 660 ° C to 770 ° C,
The time from the end of the hot rolling to the start of cooling is 30 to 120 seconds,
The moisture content in the atmosphere at the time of hot rolling is set to 70% RH or less,
And a cooling rate after hot rolling is set to 1 to 10 占 폚 / s.

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