KR20170072312A - Hot-rolled steel sheet for magnetic pole and method for manufacturing same, and rim member for hydroelectric power generation - Google Patents

Abstract

A hot-rolled steel sheet for high-strength, excellent weldability and magnetic properties, a method for producing the same, and a rim member for hydroelectric power generation. C: not less than 0.02% and not more than 0.12%, Si: not less than 0.1% and not more than 0.7%, Mn: not less than 0.8% and not more than 1.6%, P: not more than 0.03%, S: not more than 0.005%, Al: not more than 0.08% , Nb: 0.06% or more and 0.20% or less, and the balance of Fe and inevitable impurities. The Fe content is not less than 98% by area, the precipitated Fe is not more than 0.22 mass% with respect to the Fe contained in the steel, the precipitated Nb is not less than 80 mass% with respect to the Nb contained in the steel, or less of an average particle size of the carbide 6㎚, more than the yield strength in the rolling direction is 500㎫, the magnetic flux density B ₅₀ is 1.4T or more, the magnetic flux density B ₁₀₀ is the minimum value of 1.5T or more, Vickers hardness of weld heat affected zone (base material of The average value of Vickers hardness-30) or more.

Description

TECHNICAL FIELD [0001] The present invention relates to a hot-rolled steel sheet for a magnetic pole, a method of manufacturing the same, and a rim member for a hydro-

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet for a magnetic pole suitable for a rim member for hydroelectric power generation, a method of manufacturing the same, and a rim member for hydroelectric power generation.

In recent years, from the viewpoint of conservation of global environment, global warming has become a problem, and the demand for natural energy that does not emit carbon dioxide gas is increasing. In view of the suppression of global warming, hydropower generation is promising as a clean energy source in recent years. Generators such as hydro generators include a rotor and a stator, and the rotor is composed of a pole core that fulfills the role of an iron core and a rim that supports the pole core. In order to make the generating capacity, it is necessary to rotate the rotor at a high speed. Therefore, the rim is required to maintain high strength in order to withstand the centrifugal force of high-speed rotation. At the same time, the rim steel plate (rim member) is required to maintain excellent magnetic properties. Although the steel plates are welded together by welding, the welds are also required to have excellent weldability because the strength is easily varied.

Various techniques have been proposed for the hot-rolled steel sheets which have received the above and are still focused on magnetic properties and weldability.

For example, Patent Document 1 discloses that a precipitate containing Ti and V having an average particle size of less than 10 nm in a crystal grain of a ferrite phase containing a ferrite phase with an area ratio of 95% By setting the mean grain size of the ferrite phase to be in the range of 2 占 퐉 to 10 占 퐉, the yield strength in the rolling direction is not less than 700 MPa, the magnetic flux density B ₅₀ is 1.5 T or more, and B ₁₀₀ is 1.6 T Or more of the steel sheet is obtained.

In Patent Document 2, a steel sheet containing 0.05 to 0.15% of C, 0.5% or less of Si, 0.70 to 2.00% of Mn, 0.10 to 0.30% of Ti, and 0.0015 to 0.0050% of B in weight% And then rolled at 500 DEG C or lower, thereby obtaining a high-strength hot-rolled steel sheet having a high magnetic flux density.

Patent Document 3 discloses a ferrite structure containing at least one of C≤0.10% and Ti: 0.02-0.2%, furthermore Mo≤0.7% and W≤1.5%, and at least one of Ti, Mo and W High-strength, high-strength hot-rolled steel sheet having a strength of 590 MPa or higher and having a carbide of less than 10 nm dispersed therein.

International Publication No. 2013/115205 Japanese Patent Application Laid-Open No. 63-166931 Japanese Patent Application Laid-Open No. 2003-268509

However, since the technique proposed in Patent Document 1 includes solute V (solute V), the control of the amount of cementite which coarsely precipitates is remarkably difficult and the weldability deteriorates.

In the technique proposed in Patent Document 2, it is necessary to wind at a temperature range where the coiling temperature is 500 DEG C or less and the control is difficult, and there arises a problem of non-uniformity of characteristics between coils and coils. In addition, the low-temperature-transformation phase is not suitable as a member for a rim due to deterioration of the plate shape due to unevenness of transformation deformation.

In the technique proposed in Patent Document 3, not only the weldability is considered but also the influence of coarse cementite is not taken into consideration, so that stable magnetic properties can not be obtained.

In view of such circumstances, the present invention aims to provide a hot-rolled steel sheet for a magnetic pole having a yield strength in the rolling direction of 500 MPa or more and excellent weldability and magnetic properties, a method for producing the same, and a rim member for hydroelectric power generation.

As a result of intensive investigations on the requirements of a steel sheet having high strength, good weldability and good magnetic properties, it has been found that the decrease in hardness in a heat-affected zone is caused by the dissolution of carbides containing V, ) Was found to be large. In order to suppress the softening in the weld heat affected zone, it was found that the content of Si, which is a solute strengthening element, was effective after the addition of V or the control of the content thereof. On the other hand, the magnetic properties in a high magnetic field are lowered by the inclusion of Si. Therefore, as a result of studies for improving the magnetic properties, it has been found that suppressing the generation of coarse cementite to the extreme, and precipitating carbides having a good coherent relation with the matrix results in high strength and good magnetic properties It has become clear that these conditions are compatible with each other.

The present invention has been made on the basis of the above-mentioned recognition, and it is based on the following points.

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet has a composition of C: 0.02 to 0.12%, Si: 0.1 to 0.7%, Mn: 0.8 to 1.6%, P: 0.03% 0.08% or less of Al, 0.006% or less of N, 0.06% or more and 0.20% or less of Nb, the balance being Fe and incidental impurities, and the structure is such that the ferrite phase occupies 98% , The precipitated Fe is 0.22 mass% or less with respect to the Fe contained in the steel, the precipitated Nb is 80 mass% or more with respect to the amount of Nb contained in the steel, the average particle diameter of the carbide containing precipitated Nb is 6 nm or less , A yield strength in a rolling direction of 500 MPa or more, a magnetic flux density B ₅₀ of 1.4 T or more, a magnetic flux density B ₁₀₀ of 1.5 T or more, and a minimum value of Vickers hardness of a weld heat affected zone (Vickers hardness -30) < / RTI > or higher.

[2] The hot-dip coated steel sheet for stimulation as described in [1], further satisfying the following formula (1).

[3] The steel sheet according to [1] or [2] above, which further comprises at least one of V: at least 0.01% and less than 0.05%, and Ti: And the hot-rolled steel sheet for stimulation.

[4] A steel material having the composition described in any one of [1] to [3] above is heated to a temperature of not lower than 1100 ° C. and not higher than 1350 ° C., followed by rough rolling at a temperature of not lower than 1100 ° C. And then subjected to hot rolling at a finishing rolling temperature of 840 DEG C or higher. After cooling at an average cooling rate of 30 DEG C / s or higher within 3 seconds after finishing rolling, Wherein the hot-rolled steel sheet is coiled.

[5] The method for manufacturing a hot-rolled steel sheet for magnetic pole according to [4], wherein the surface of the steel sheet is further plated.

[6] The method as described in [5], wherein the plating treatment is any one of a hot-dip galvanizing treatment, a hot-dip galvanizing treatment, and an electrogalvanized plating treatment Wherein the hot-rolled steel sheet has a thickness of 10 mm or less.

[7] The method according to the above [5] or [6], wherein the composition of the plating layer formed in the plating treatment includes one or more of Zn, Si, Al, A method for manufacturing a hot - rolled steel sheet.

[8] A rim member for hydroelectric power generation comprising the hot-rolled steel sheet for magnetic pole according to any one of [1] to [3].

In the present invention, the hot-rolled steel sheet for magnetic pole refers to a hot rolled steel sheet for which no plating treatment (hot rolled steel plate), hot dip galvanized steel sheet (GI), galvanized steel sheet after further galvanizing treatment ) And electro-galvanized (EG).

According to the present invention, it is possible to obtain a hot-rolled steel sheet for magnetic pole having a yield strength in the rolling direction of 500 MPa or more and excellent weldability and magnetic properties. The hot-rolled steel sheet for magnetic pole of the present invention is suitable for a hydroelastic rim member or the like. The use of the hot-rolled steel sheet for stimulation of the present invention in the hydro-power generation rim member can realize the high efficiency of hydroelectric power generation and the improvement of the service life, and the effect is remarkable.

(Mode for carrying out the invention)

Hereinafter, the present invention will be described in detail. The following percentages are by weight unless otherwise specified.

First, an organization which is an important requirement of the steel sheet of the present invention will be described.

Area ratio of ferrite phase: 98% or more (including 100%)

When the dislocation density is large, the magnetic flux density remarkably decreases. Therefore, it is necessary to make a structure that does not include a low-temperature transformation phase such as a bainite phase or a martensite phase that contains much dislocation density. In the present invention, in order to satisfy the desired magnetic properties, the area ratio of the ferrite phase is set to 98% or more. The remainder may include bainite phase, martensite phase and pearlite.

The precipitated Fe is not more than 0.22% of the Fe contained in the steel

Fe as a precipitate is derived from cementite. Since coarse cementite causes the magnetic flux density to be lowered, it is preferable to reduce the magnetic flux density as much as possible. In order to reduce the cementite to obtain the magnetic flux density required by the present invention, the ratio of the "amount of Fe precipitated" to the "amount of Fe contained in steel" (hereinafter sometimes referred to as the Fe precipitation amount) is 0.22% or less Needs to be. And preferably not more than 0.20%. The Fe precipitation amount can be measured by the method described in the following Examples.

In order to suppress cementite formation, it is preferable that the amount of C contained is precipitated as carbide containing Nb as much as possible. Therefore, it is preferable to satisfy the following expression (1).

The expression (1) above indicates that C is precipitated as a fine carbide by binding with Nb, V and / or Ti to reduce cementite from the viewpoint of chemical composition when the production conditions are appropriate. , The deposition amount of the cementite becomes a range in which the magnetic flux density is not lowered. The magnetic flux rate B ₅₀ of 1.5 T or more and the magnetic flux density B ₁₀₀ of 1.6 T or more are obtained by setting the expression (1) to 0.03 or less. On the other hand, since C forms a fine carbide, the formula (1) is preferably -0.005 or more.

In addition, N, V, and C that do not bond with Ti precipitate as Fe carbide. In order to precipitate almost all of C contained in the steel as fine carbides including Nb, V and Ti, rough rolling before completion of the finish rolling is preferably completed at 1100 占 폚 or higher.

The amount of precipitated Nb is preferably 80% by mass or more relative to the amount of Nb contained in the steel

In the present invention, high strength having a yield strength of 500 MPa or more can be obtained by dispersing a carbide containing fine Nb. When the ratio of the "amount of precipitated Nb" to the "amount of Nb contained in the steel" (sometimes referred to as Nb precipitation amount or Nb precipitation ratio) is less than 80%, desired strength can not be obtained, The magnetic flux density decreases due to the influence of Nb. From the above viewpoint, the Nb precipitation amount is 80% or more. It is preferably at least 85%. The Nb precipitation amount can be measured by the method described in the following Examples.

The average particle diameter of the carbide containing precipitated Nb is 6 nm or less

The amount of the strength that is increased by dispersing the carbide containing Nb rises with the decrease of the carbide particle diameter. In order to obtain a high strength having a yield strength of 500 MPa or more, it is necessary that the carbide including precipitated Nb has an average grain size of 6 nm or less. The average particle diameter of the carbide can be measured by the method described in the following Examples.

Next, reasons for limiting the composition of the present invention will be described.

C: not less than 0.02% and not more than 0.12%

C combines with Nb to form a fine carbide containing Nb, thereby contributing to the strengthening of the steel sheet. In order to obtain a yield strength of 500 MPa or more, at least C should be contained in an amount of 0.02% or more. It is preferably 0.03% or more. On the other hand, the content exceeding 0.12% generates cementite and lowers the magnetic flux density. Therefore, the upper limit of C is 0.12%. It is preferably 0.10% or less.

Si: 0.1% or more and 0.7% or less

Si has the effect of suppressing the softening of the weld heat affected zone as a solid solution strengthening element against heat. Further, there is an effect of making the cementite finer and restraining adverse effects of a decrease in magnetic flux density due to cementite precipitation. Thus, Si is an important requirement in the present invention. The lower limit of Si for achieving this effect is 0.1%. Or more, preferably 0.2% or more, and more preferably 0.35% or more. On the other hand, if the Si content exceeds 0.7%, the adverse effect of the decrease of the magnetic flux density due to the Si content is noticeable, and a red scale is generated on the surface of the steel sheet to deteriorate the appearance, do. From the above, the upper limit amount of Si is set to 0.7%. Preferably, it is 0.6% or less.

Mn: not less than 0.8% and not more than 1.6%

The carbide containing Nb becomes fine with the lowering of the transformation temperature from austenite to ferrite. Since Mn has an effect of lowering the transformation temperature from austenite to ferrite, the inclusion of Mn makes the carbide including Nb finer and higher in strength. In order to obtain a yield strength of 500 MPa or more, Mn should be 0.8% or more. On the other hand, if it exceeds 1.6%, a bainite phase tends to be generated, which causes a decrease in strength due to the formation of coarse cementite and a difference in magnetic flux density. From the above, the range of the Mn content is set to 0.8% or more and 1.6% or less. , Preferably not less than 0.9% and not more than 1.5%.

P: not more than 0.03%

P is an element segregating at the grain boundary and remarkably deteriorating the toughness of the welded portion. Therefore, it is preferable that P is reduced as much as possible. In the present invention, the P content is set to 0.03% or less in order to avoid the above problem. It is preferably 0.02% or less.

S: not more than 0.005%

S exists as an inclusion of MnS in the steel. This inclusion is a factor of lowering the magnetic flux density at the coarse point. Therefore, in the present invention, it is preferable to reduce the S content as much as possible, and it is made 0.005% or less. It is preferably 0.003% or less.

Al: 0.08% or less

When Al is contained as a deoxidizer at the stage of steelmaking, it contains 0.02% or more. On the other hand, when the Al content exceeds 0.08%, magnetic flux density decreases due to coarse inclusions such as alumina. Therefore, the Al content should be 0.08% or less. Preferably 0.07% or less.

N: not more than 0.006%

N combines with Nb to form a coarse nitride, which causes a decrease in the magnetic flux density. In addition, the deposition amount of fine carbides including Nb contributing to strengthening is reduced, which leads to a decrease in strength. Therefore, it is preferable to reduce the N content as much as possible, and the upper limit amount is set to 0.006%. It is preferably 0.005% or less.

Nb: not less than 0.06% and not more than 0.20%

Nb is an element contributing to the strengthening of the steel sheet by forming fine carbides. In order to obtain a yield strength of 500 MPa or more, the Nb content needs to be 0.06% or more. On the other hand, if it exceeds 0.20%, the carbide including the coarse Nb can not be dissolved during the heating of the slab before hot rolling, and the contribution to high strength is saturated and the magnetic flux density is decreased. From the above, the range of Nb content is 0.06% or more and 0.20% or less. , Preferably not less than 0.08% and not more than 0.18%. A suitable range for obtaining the yield strength in the rolling direction of 550 MPa or more is 0.10% or more and 0.18% or less.

The remainder is Fe and inevitable impurities.

In addition to the above-mentioned component composition, in addition to the above-mentioned composition, at least one of V: not less than 0.01% and not more than 0.05%, Ti: not less than 0.01% and not more than 0.05% can do.

V and Ti combine with C to contribute to new high strength. In order to obtain this effect, it is preferable that both V and Ti are contained by 0.01% or more. On the other hand, when V is contained in an amount of 0.05% or more, the effect of softening due to dissolution of carbide including V in the weld heat affected zone becomes present, and weldability lowers. When Ti is contained in an amount of not less than 0.05%, carbides including Ti are left in the slab heating step before hot rolling, which causes a decrease in the magnetic flux density. From the above, when contained, V is not less than 0.01% and not more than 0.05%, and Ti is not less than 0.01% and not more than 0.05%. Preferably, V is not less than 0.01% and not more than 0.04%, and Ti is not less than 0.01% and not more than 0.03%.

Next, reasons for limiting the characteristics of the hot-rolled steel sheet for magnetic pole according to the present invention will be described.

When the yield strength in the rolling direction is 500 MPa or more

When used as a rim member for hydroelectric power generation, strength is required. If the yield strength in the rolling direction is 500 MPa or more, the sheet thickness can be reduced, and it becomes possible to apply it to the rim member for hydroelectric power generation with high efficiency. In this case, the yield strength in the rolling direction is specified because the yield strength by the tensile test in the direction parallel to the rolling direction becomes important. The yield strength of the present invention is particularly suitable for steel sheets up to 700 MPa.

A magnetic flux density B ₅₀ of 1.4 T or more, a magnetic flux density B ₁₀₀ of 1.5 T or more

When the magnetic flux density B ₅₀ is 1.4 T or more and the magnetic flux density B ₁₀₀ is 1.5 T or more, high efficiency of hydroelectric power generation can be achieved in the case of being used in a rim member for hydroelectric power generation.

The lowest value of the Vickers hardness of the weld heat affected zone (the average value of the Vickers hardness of the base material is -30) or more

Most of the rim members for hydroelectric power generation are welded together. By making the Vickers hardness of the weld heat affected zone (the average value of the Vickers hardness of the base material is -30) or more, defects and problems at the welded portion can be suppressed. The welding conditions at this time may be the same as those described in the embodiment.

Next, a method for manufacturing a hot-rolled steel sheet for magnetic pole according to the present invention will be described.

In the hot-rolled steel sheet for magnetic pole according to the present invention, the steel material (steel slab) having the above-described composition is heated to 1100 DEG C or higher and 1350 DEG C or lower, followed by rough rolling to a temperature of 1100 DEG C or higher and the finish rolling temperature is 840 DEG C or higher And cooling the steel sheet at an average cooling rate of 30 占 폚 / s or higher within 3 seconds after completion of finish rolling, and then winding the steel sheet at 550 占 폚 or higher and 700 占 폚 or lower.

In the present invention, the method of the solvent of the steel is not particularly limited, and a known solvent method such as a converter, an electric furnace or the like can be adopted. Further, secondary refining may be performed in a vacuum degassing furnace. Thereafter, it is preferable to make the slab (steel material) by the continuous casting method in view of the problem of productivity and quality. However, it is preferable to use a casting method such as ingot casting and blooming, a thin-slab continuous casting method, The slab may be formed by a known casting method.

Heating temperature of steel material: 1100 ℃ or more and 1350 ℃ or less

It is necessary to heat the steel material prior to hot rolling to obtain a substantially homogeneous austenite phase. If the heating temperature is lower than 1100 占 폚, coarse carbides including Nb and Ti can not be dissolved and the strength and magnetic flux density are lowered. On the other hand, if the heating temperature exceeds 1350 deg. C, the scale production amount increases, the scale is inherited during the hot rolling, and the surface properties of the hot-rolled steel sheet deteriorate. Therefore, the heating temperature of the steel material is set to 1100 ° C or more and 1350 ° C or less. Preferably 1150 DEG C or more and 1300 DEG C or less. However, when hot rolling the steel material, if the steel material after casting is in a temperature range of 1100 ° C or more and 1350 ° C or less, or if the carbonaceous material of the steel material is dissolved, the steel material is directly heated Hot direct rolling may be used.

Rolling at a temperature of 1100 占 폚 or more to finish the hot rolling at a finish rolling temperature of 840 占 폚 or higher

N, V, and C that do not bond with Ti precipitate as Fe carbide. In order to precipitate almost all of C contained in the steel as fine carbides including Nb, V and Ti, rough rolling before finish rolling must be completed at 1100 占 폚 or higher. When the rough rolling is completed at less than 1100 DEG C, carbides including Nb, V and Ti are precipitated in the austenite by using the deformation introduced by rough rolling as a driving force and maintained for a long time after that, This is due to the fact that the adverse effect on density is made present. When the finishing rolling temperature is lower than 840 캜, ferrite transformation starts during finish rolling, and the ferrite lips are expanded. Since a large amount of potential is introduced into the inside of the expanded ferrite lips, the magnetic flux density is reduced. Therefore, the finishing rolling temperature should be 840 캜 or higher. Preferably 860 DEG C or more. The temperature in the finish rolling is less than 1100 占 폚. However, in the tandem rolling mill in the finish rolling, there is no time to deposit and grow compared with the rough rolling, so that the bad influence at the time of rough rolling does not become present.

Cooling at an average cooling rate of 30 DEG C / s or more within 3 seconds after finishing rolling

The carbide containing Nb is refined as the transformation temperature from the austenite to the ferrite is lowered. In order to obtain a carbide having an average grain size of 6 nm or less, the transformation temperature from austenite to ferrite needs to be 700 DEG C or less. For this purpose, it is necessary to cool the steel sheet at an average cooling rate of 30 DEG C / s or more within 3 seconds after completion of finish rolling. The average cooling rate is an average cooling rate from the finish rolling temperature to 700 占 폚.

Coiling temperature: 550 캜 to 700 캜

If the coiling temperature exceeds 700 DEG C, the carbide coarsens and desired strength and magnetic properties are not obtained. On the other hand, when the temperature is lower than 550 DEG C, a bainite phase is generated, and magnetic properties are lowered. From the above, the range of the coiling temperature is set to 550 占 폚 or more and 700 占 폚 or less. The temperature is preferably 580 DEG C or higher and 680 DEG C or lower.

Thus, the hot-rolled steel sheet for magnetic pole of the present invention is produced. Further, even if the hot-rolled steel sheet for magnetic pole of the present invention is put in a continuous hot-dip plating line having an annealing temperature of 720 占 폚 or lower, the material is not affected. Therefore, the surface of the steel sheet can be further plated to have a plated layer on the surface of the steel sheet. In addition, since the material does not affect the plating process or the composition of the plating bath, the plating process can be applied to any of hot dip galvanizing, galvannealing hot dip galvanizing, and electro galvanizing. The composition of the plating bath may be one containing at least one of Zn, Si, Al, Ni and Mg. That is, the composition of the plating layer formed in the plating treatment may include one or more of Zn, Si, Al, Ni, and Mg.

The hot-rolled steel sheet for a magnetic pole according to the present invention obtained by the above-described method is suitable for parts requiring a high magnetic pole, and is particularly suitable for use as a rim member for hydroelectric power generation. For example, the hot-rolled steel sheet of the present invention can be cut and cut into a predetermined shape by means of shearing, punching, laser cutting or the like and used as an electronic member for a rim or a core (pole core or the like). In particular, the hot-rolled steel sheet of the present invention can be suitably applied to a generator rim that requires both high strength and good magnetic properties. When the steel sheet is laminated, it is preferable to electrically insulate the steel sheet from the laminated steel sheet, for example, by applying an insulating coating to the steel sheet or sandwiching the insulating material therebetween.

Example 1

Hot-rolled steel sheets were produced under the hot rolling conditions shown in Table 2 for steel materials having the composition shown in Table 1 and having a thickness of 250 mm. For some of them, a galvannealing hot dip galvanizing treatment was further carried out. The alloying hot-dip plating treatment was performed in a continuous hot-dip coating line having an annealing temperature of 700 ° C or lower, a molten bath composition of Zn-0.13 mass% Al, a temperature of the plating bath of 460 ° C, and an alloying temperature of 530 ° C , And the coating weight was 45 to 65 g / m 2 per one side.

A test piece was taken from the hot-rolled steel sheet or the alloyed hot-dip galvanized steel sheet having the plate thickness of 1.6 mm to 3.2 mm obtained above, and the structure was observed by the following method to evaluate the performance.

(I) Tissue observation

The area ratio of each phase was evaluated by the following technique. A cross section parallel to the rolling direction was cut out from the hot-rolled steel sheet or the alloyed hot-dip galvanized steel sheet as an observation surface to etch the metal structure of the central portion of the plate to 3% or nital, I took a field of view by magnifying it. The ferrite phase is a structure that has no form of corrosion marks or cementite in the grain. The bainite phase, martensite phase, pearlite, and the like other than the ferrite phase were separated by image analysis to obtain an area ratio to the observation field. In seeking the area, the ferrite system was regarded as part of the ferrite phase.

The average particle size of the carbide containing precipitated Nb was observed at 135,000 times or more using a transmission electron microscope to obtain an average of the particle diameters of carbides of 100 or more and a circle equivalent diameter was obtained as the particle size of each carbide .

The amount of precipitation of Fe was about 0.2 g in a 10% AA-based electrolytic solution (10 vol% acetylacetone-1 mass% tetramethylammonium chloride-methanol) at a current density of 20 mA / cm2 and then filtered from the electrolytic solution to precipitate And the amount of Fe contained in the precipitate was determined by the ICP-MS method, and the ratio of the amount of Fe contained in the precipitate to the mass of the substrate iron transferred by the constant current electrolysis was obtained.

The Nb precipitation amount (Nb precipitation ratio) was subjected to constant current electrolysis in the same manner as the method for measuring the Fe precipitation amount, and the amount of Nb contained in the electrolytic solution was measured by the ICP-MS method. The amount of Nb contained in the electrolytic solution was the amount of Nb in the solid state, and the amount of Nb precipitated was obtained by subtracting the amount of Nb in the solid state from the amount of Nb.

(Ii) Tensile test

JIS No. 5 tensile test specimens were prepared from the hot-rolled steel sheet or the alloyed hot-dip galvanized steel sheet in the direction parallel to the rolling direction and subjected to a tensile test according to JIS Z 2241 (2011) five times to obtain an average yield strength (YS) Tensile strength (TS) and total elongation (El) were obtained. The crosshead speed of the tensile test was 10 mm / min.

(Iii) Magnetic flux density measurement

A sample of 30 mm x 280 mm was taken from the hot-rolled steel sheet or the alloyed hot-dip galvanized steel sheet, and the magnetic flux density B ₅₀ and the magnetic flux density B ₁₀₀ were obtained by measurement according to JIS C 2555 using a DC magnetic property measuring apparatus. B ₅₀ and B ₁₀₀ show magnetic flux densities at magnetizing forces of 5000 A / m and 10000 A / m, respectively.

(Iv) Weldability evaluation

As a welding test, carbon dioxide gas arc welding using a wire having a diameter of 1.2 mm was performed and evaluated. Welding conditions are butt welding with a welding speed of 80 cm / min, a welding current of 220 A, a welding voltage of 25 V, and a gap of 1 mm. After the welding, the bead end face was cut out, and the Vickers hardness test of a test load of 0.49 N was carried out with respect to the direction crossing the welded portion at intervals of 0.5 mm in the central portion of the plate thickness. On the other hand, the hardness of the base material was taken as the average value measured at 5 points from a test load of 0.49 N at a position 30 mm or more away from the welded portion. Table 3 shows the difference between the hardness of the base material (average value of the base material hardness) and the minimum hardness at the heat affected zone (the minimum value of the hardness of the weld heat affected zone).

Table 3 shows the results obtained by the above.

All of the present inventions have weldability in which the yield strength YS in the rolling direction is not less than 500 MPa and the minimum value of the Vickers hardness of the weld heat affected zone is not less than (average value of Vickers hardness of the base material is -30), and the magnetic flux density B ₅₀ 1.4 T or more and a magnetic flux density B ₁₀₀ of 1.5 T or more, which is excellent in magnetic properties, can be obtained from the hot-rolled steel sheet (alloyed hot-dip galvanized steel sheet). On the other hand, in Comparative Examples outside the scope of the present invention, at least one of yield strength, weldability, and magnetic properties is poor.

Claims (8)

Wherein the composition of C is 0.02 to 0.12%, Si is 0.1 to 0.7%, Mn is 0.8 to 1.6%, P is 0.03% or less, S is 0.005% or less, Al is 0.08% % Or less, N: 0.006% or less, Nb: 0.06% or more and 0.20% or less, the balance being Fe and inevitable impurities,
In the structure, the area ratio of the ferrite phase is 98% or more,
The precipitated Fe is contained in an amount of not more than 0.22 mass% with respect to the Fe contained in the steel, the precipitated Nb is not less than 80 mass% with respect to the amount of Nb contained in the steel, the carbide including precipitated Nb has an average particle size of 6 nm or less,
The minimum value of the Vickers hardness of the weld heat affected zone (the average value of the Vickers hardness of the base material is -30) or more, the yield strength in the rolling direction is 500 MPa or more, the magnetic flux density B ₅₀ is 1.4 T or more, the magnetic flux density B ₁₀₀ is 1.5 T or more Hot rolled steel sheet for stimulation. The method according to claim 1,
And further satisfies the following formula (1).

3. The method according to claim 1 or 2,
Wherein at least one of V: at least 0.01% and less than 0.05%, and Ti: at least 0.01% and less than 0.05%, in mass%, in addition to the above composition. A steel material having a composition according to any one of claims 1 to 3 is heated to a temperature of not lower than 1100 DEG C and not higher than 1350 DEG C and then subjected to rough rolling at a temperature of not lower than 1100 DEG C and a finish rolling temperature of 840 DEG C or higher And cooling the steel sheet at an average cooling rate of 30 DEG C / s or higher within 3 seconds after finishing rolling, and then winding the steel sheet at a temperature of 550 DEG C to 700 DEG C . 5. The method of claim 4,
And the surface of the steel sheet is further plated. 6. The method of claim 5,
Wherein the plating treatment is any one of a hot dip galvanizing treatment, an alloying hot dip galvanizing treatment, and an electro galvanizing treatment. The method according to claim 5 or 6,
Wherein the composition of the plating layer formed in the plating treatment includes one or more of Zn, Si, Al, Ni, and Mg. A rim member for hydroelectric power generation comprising the hot-rolled steel sheet for magnetic pole according to any one of claims 1 to 3.