JP6020769B1 - Hot-rolled steel sheet for magnetic pole, method for producing the same, and rim member for hydroelectric power generation - Google Patents

Abstract

A hot-rolled steel sheet for magnetic poles having high strength and excellent weldability and magnetic properties, a method for producing the same, and a rim member for hydroelectric power generation are provided. C: 0.02% to 0.12%, Si: 0.1% to 0.7%, Mn: 0.8% to 1.6%, P: 0.03% or less, S: 0.005% or less, Al: 0.08% or less, N: 0.006% or less Nb: 0.06% or more and 0.20% or less, with the balance being Fe and inevitable impurities. The ferrite phase is 98% or more by area ratio, the precipitated Fe is 0.22% by mass or less with respect to the amount of Fe contained in the steel, and the precipitated Nb is 80% by 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, the yield strength in the rolling direction is 500 MPa or more, the magnetic flux density B50 is 1.4 T or more, the magnetic flux density B100 is 1.5 T or more, the Vickers hardness of the weld heat affected zone Is the minimum value (average value of Vickers hardness of base material minus 30).

Description

  The present invention relates to a hot-rolled steel sheet for magnetic poles suitable for a rim member for hydroelectric power generation, a manufacturing method thereof, and a rim member for hydroelectric power generation.

  In recent years, global warming has been seen as a problem from the viewpoint of conservation of the global environment, and demand for natural energy that does not emit carbon dioxide gas has increased. And from the viewpoint of suppressing such global warming, recently, hydroelectric power generation is promising as a clean energy source. A generator such as a hydroelectric generator includes a rotor and a stator, and the rotor includes a pole core that serves as an iron core and a rim that supports the core. In order to earn power generation capacity, it is necessary to rotate the rotor at high speed. Therefore, the rim is required to maintain a 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 have excellent magnetic properties. And although steel plates are joined by welding, since a weld part is easy to change intensity | strength, it is also requested | required that it is excellent in weldability (weldability).

  In response to the above, various techniques have been proposed for hot-rolled steel sheets focusing on magnetic properties and weldability.

For example, Patent Document 1 includes a ferrite phase including a ferrite phase having an area ratio of 95% or more, and containing Ti and V having an average particle size of less than 10 nm in crystal grains of the ferrite phase. ) And the average grain size of the ferrite phase is in the range of 2 μm or more and less than 10 μm, the yield strength in the rolling direction is 700 MPa or more, and the magnetic flux density B ₅₀ is 1.5 T. As described above, it is said that a steel sheet having an electromagnetic property of B ₁₀₀ of 1.6 T or more can be obtained.

  In Patent Document 2, hot rolling a steel sheet containing C: 0.05 to 0.15%, Si: 0.5% or less, Mn: 0.70 to 2.00%, Ti: 0.10 to 0.30%, B: 0.0015 to 0.0050% by weight%. After that, a high-tensile hot-rolled steel sheet having a high magnetic flux density is obtained by winding at 500 ° C. or lower.

  Patent Document 3 includes C ≦ 0.10%, Ti: 0.02 to 0.2%, and further includes at least one of Mo ≦ 0.7% and W ≦ 1.5%. The ferrite structure substantially includes at least Ti, Mo, and W. A high workability high-strength hot-rolled steel sheet for a rotator iron core having a strength of 590 MPa or higher is disclosed.

International Publication No.2013 / 115205 JP-A-63-166931 JP2003-268509A

  However, since the technique proposed in Patent Document 1 includes solute V, it is extremely difficult to control the amount of cementite that precipitates coarsely, and the weldability is further deteriorated.

  In the technique proposed in Patent Document 2, it is necessary to wind the coil in a temperature range where the coiling temperature is 500 ° C. or less, which is difficult to control, and variations in characteristics between coils and within the coil become a problem. Furthermore, the low temperature transformation phase is not suitable as a rim member due to deterioration of the plate shape due to non-uniform transformation strain.

  In the technique proposed in Patent Document 3, not only the weldability is not considered, but also the influence of coarse cementite is not considered, so that stable magnetic characteristics cannot be obtained.

  In view of such circumstances, an object of the present invention is to provide a hot-rolled steel sheet for magnetic poles having excellent weldability and magnetic properties, yield strength in the rolling direction: 500 MPa or more, a manufacturing method thereof, and a rim member for hydroelectric power generation. To do.

  As a result of intensive investigations on the requirements of steel sheets that combine high strength with good weldability and good magnetic properties, the hardness reduction in the heat-affected zone is due to the dissolution of carbides containing V with high solubility (re- solution)) In order to suppress softening in the weld heat affected zone, it was found that the addition of Si, which is a solute strengthening element, is effective after adding V or controlling the content. did. On the other hand, the magnetic properties in a high magnetic field deteriorate due to the inclusion of Si. Therefore, as a result of investigations to improve magnetic properties, it is possible to suppress the formation of coarse cementite to the limit and to precipitate carbide with a good coherent relation with the matrix. It was revealed that this is a requirement to achieve both good magnetic properties.

This invention is made | formed based on the above knowledge, and makes the following a summary.
[1] Component composition is mass%, C: 0.02% to 0.12%, Si: 0.1% to 0.7%, Mn: 0.8% to 1.6%, P: 0.03% or less, S: 0.005% or less, Al: 0.08% or less, N: 0.006% or less, Nb: 0.06% or more and 0.20% or less, the balance is Fe and incidental impurities, and the structure has a ferrite phase area ratio of 98% or more The precipitated Fe is 0.22% by mass or less based on the amount of Fe contained in the steel, and the precipitated Nb is 80% by mass or more based on the amount of Nb contained in the steel. diameter is not more 6nm or less, the rolling direction yield strength of the (rolling direction) is more than 500 MPa, the magnetic flux density B ₅₀ is more than 1.4 T, the magnetic flux density B ₁₀₀ is over 1.5T, the minimum value of Vickers hardness of the weld heat affected zone Is a hot-rolled steel sheet for magnetic poles, characterized in that it is (average value of Vickers hardness of base material -30) or more.
[2] The hot-rolled steel sheet for magnetic poles according to [1], further satisfying the following formula (1).

[3] The above-mentioned [1] or [2], wherein, in addition to the component composition, one or more of V: 0.01% to less than 0.05% and Ti: 0.01% to less than 0.05% are contained by mass% A hot-rolled steel sheet for magnetic poles according to 1.
[4] A steel material having the composition according to any one of [1] to [3] above is heated at a temperature of 1100 ° C. or higher and 1350 ° C. or lower, and then roughly rolled at a temperature of 1100 ° C. or higher ( After rough rolling is completed, hot rolling is performed at a finishing rolling temperature of 840 ° C or higher, and after cooling at an average cooling rate of 30 ° C / s or higher within 3 seconds after finishing rolling, 550 ° C or higher A method for producing a hot-rolled steel sheet for magnetic poles, which is coiled at a temperature of 700 ° C. or lower.
[5] The method for producing a hot-rolled steel sheet for magnetic poles according to the above [4], wherein the steel sheet surface is further plated.
[6] The above described plating process is any one of a hot-dip galvanizing process, an alloyed hot-dip galvannealing process, and an electrogalvanized plating process. The manufacturing method of the hot-rolled steel plate for magnetic poles as described in [5].
[7] The composition of 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, Ni, and Mg. Manufacturing method of hot-rolled steel sheet for magnetic pole.
[8] A hydroelectric rim member comprising the hot-rolled steel sheet for magnetic poles according to any one of [1] to [3].
In the present invention, the hot-rolled steel sheet for magnetic poles is not subjected to plating treatment (hot-rolled steel sheet), subjected to hot-dip galvanizing treatment (GI), and further subjected to alloying treatment after hot-dip galvanizing treatment. Any of the above (GA) and electrogalvanized (EG) are also targeted.

  According to the present invention, a hot rolled steel sheet for magnetic poles having a yield strength in the rolling direction of 500 MPa or more and excellent weldability and magnetic properties can be obtained. The hot-rolled steel sheet for magnetic poles of the present invention is suitable for a rim member for hydroelectric power generation. By using the hot-rolled steel sheet for magnetic poles of the present invention for the rim member for hydroelectric power generation, it is possible to achieve high efficiency of hydroelectric power generation and improvement of equipment life, and the effect is remarkable.

  Hereinafter, the present invention will be described in detail. In addition, the following% shall mean the mass% unless there is particular notice.

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

Ferrite phase area ratio: 98% or more (including 100%)
When the dislocation density is high, the magnetic flux density is significantly reduced. For this reason, it is necessary to have a structure that does not include a low-temperature transformation phase (dislocation density) such as a bainite phase or a martensite phase containing a large amount of dislocation density. In the present invention, in order to satisfy desired magnetic characteristics, the area ratio of the ferrite phase is set to 98% or more. The balance includes bainite phase, martensite phase and pearlite.

Precipitated Fe is 0.22% or less with respect to the amount of Fe contained in steel. Fe as a precipitate is derived from cementite. Since coarse cementite causes a decrease in magnetic flux density, it is desirable to reduce it as much as possible. In order to reduce the cementite and obtain the magnetic flux density required in the present invention, the ratio of the “precipitated Fe amount” to the “Fe amount contained in the steel” (hereinafter also referred to as Fe precipitation amount) is 0.22. Must be less than%. Desirably, it is 0.20% or less. In addition, the amount of Fe precipitation can be measured by the method described in the examples described later.

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

The above formula (1) indicates that C is combined with Nb, V and / or Ti and precipitated as fine carbides to reduce cementite from the viewpoint of chemical composition when the production conditions are appropriate. It is a type | formula, By making it 0.040 or less, the precipitation amount of cementite becomes the range which does not reduce magnetic flux density. The magnetic flux density B ₅₀ is more than 1.5T, especially excellent magnetic properties flux density B ₁₀₀ is equal to or greater than 1.6T can be obtained by a 0.03 or less (1). On the other hand, since C forms fine carbides, it is desirable that the formula (1) is −0.005 or more.

  Further, C that does not bind to Nb, V, and Ti precipitates as Fe carbide. In order to precipitate almost all of the contained C as fine carbides containing Nb, V, and Ti, it is preferable to complete rough rolling before finish rolling at 1100 ° C. or higher.

The amount of precipitated Nb is 80% by mass or more with respect to the amount of Nb contained in the steel. In the present invention, a high strength having a yield strength of 500 MPa or more can be obtained by dispersing carbide containing fine Nb. If the ratio of the “precipitated Nb amount” to the “Nb amount contained in steel” (sometimes referred to as Nb precipitation amount or Nb precipitation ratio) is less than 80%, the desired strength cannot be obtained, and Magnetic flux density decreases due to the effect of molten Nb. From the above viewpoint, the Nb precipitation is 80% or more. Desirably, it is 85% or more. The amount of Nb deposited can be measured by the method described in the examples described later.

Average particle size of precipitated carbide containing Nb is 6nm or less
The amount of strength that increases by dispersing carbide containing Nb increases as the carbide particle diameter decreases. In order to obtain a high strength with a yield strength of 500 MPa or more, the average particle diameter of the precipitated Nb-containing carbide needs to be 6 nm or less. In addition, the average particle diameter of a carbide | carbonized_material can be measured by the method as described in the Example mentioned later.

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

C: 0.02% to 0.12%
C is an element that forms fine carbides containing Nb by combining with Nb and contributes to the enhancement of the strength of the steel sheet. In order to obtain a yield strength of 500 MPa or more, at least C must be contained by 0.02% or more. Preferably it is 0.03% or more. On the other hand, a content exceeding 0.12% generates cementite and lowers the magnetic flux density. Therefore, the upper limit of C is set to 0.12%. Preferably it is 0.10% or less.

Si: 0.1% to 0.7%
Si is a solid solution strengthening element that is stable against heat and has an effect of suppressing softening of the weld heat affected zone. Furthermore, there is an effect that the cementite is refined and the adverse effect of the decrease in magnetic flux density due to the cementite precipitation is suppressed. Thus, Si is an important requirement in the present invention. The lower limit of Si for obtaining these effects is 0.1%. Preferably it is 0.2% or more, more preferably 0.35% or more. On the other hand, when the Si content exceeds 0.7%, the adverse effect of the decrease in magnetic flux density due to the Si content becomes obvious, and a red scale is generated on the surface of the steel sheet, thereby deteriorating the appearance and reducing the plating performance. From the above, the upper limit of Si is 0.7%. Preferably, it is 0.6% or less.

Mn: 0.8% to 1.6%
The carbide containing Nb becomes finer as the transformation temperature from austenite to ferrite decreases. Since Mn has the effect of lowering the transformation temperature from austenite to ferrite, the inclusion of Mn makes the carbide containing Nb finer and higher in strength. Yield strength: Mn needs to be 0.8% or more to obtain 500 MPa or more. On the other hand, when it exceeds 1.6%, a bainite phase is likely to be generated, which causes a decrease in strength and a variation in magnetic flux density due to the formation of coarse cementite. From the above, the range of Mn content is 0.8% or more and 1.6% or less. Preferably they are 0.9% or more and 1.5% or less.

P: 0.03% or less
P is an element that segregates at the grain boundary and significantly deteriorates the toughness of the weld. Therefore, it is preferable to reduce P as much as possible. In the present invention, in order to avoid the above problem, the P content is set to 0.03% or less. Preferably it is 0.02% or less.

S: 0.005% or less
S exists as inclusions such as MnS in steel. Since this inclusion is coarse, it causes a decrease in magnetic flux density. Therefore, in the present invention, it is preferable to reduce the S content as much as possible, and it is 0.005% or less. Preferably it is 0.003% or less.

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

N: 0.006% or less
N combines with Nb to form coarse nitrides, which causes a decrease in magnetic flux density. In addition, the amount of precipitation of fine carbides containing Nb that contributes to strengthening decreases, leading to a decrease in strength. Therefore, the N content is preferably reduced as much as possible, and the upper limit is set to 0.006%. Preferably it is 0.005% or less.

Nb: 0.06% to 0.20%
Nb is an element that forms fine carbides and contributes to increasing the strength of the steel sheet. In order to obtain a yield strength of 500 MPa or more, the Nb content must be 0.06% or more. On the other hand, if it exceeds 0.20%, the carbide containing coarse Nb cannot be dissolved during slab heating before hot rolling, so that not only the contribution to high strength is saturated but also the magnetic flux density is reduced. From the above, the range of Nb content is set to 0.06% or more and 0.20% or less. Preferably it is 0.08% or more and 0.18% or less. A preferred range for obtaining a yield strength in the rolling direction of 550 MPa or more is 0.10% or more and 0.18% or less.

  The balance is Fe and inevitable impurities.

  The above is the component composition in the present invention. In addition to the above-described component composition, according to the following purposes, V: 0.01% or more and less than 0.05%, Ti: 0.01% or more and less than 0.05% can do.

  V and Ti are elements that combine with C and contribute to further strengthening. In order to acquire this effect, it is preferable to contain both V and Ti 0.01% or more. On the other hand, when V is contained in an amount of 0.05% or more, the influence of softening due to dissolution of carbide containing V in the welding heat-affected zone becomes obvious, and the weldability decreases. When Ti is contained in an amount of 0.05% or more, coarse carbides containing Ti remain in the slab heating step before hot rolling, which causes a decrease in magnetic flux density. From the above, when contained, V: 0.01% or more and less than 0.05%, Ti: 0.01% or more and less than 0.05%. Preferably, V: 0.01% to 0.04%, Ti: 0.01% to 0.03%.

  Next, the reason for limiting the characteristics of the hot-rolled steel sheet for magnetic poles of the present invention will be described.

Strength is required when used in a rim member for hydroelectric power generation having a yield strength in the rolling direction of 500 MPa or more. If the yield strength in the rolling direction is 500 MPa or more, it is possible to reduce the plate thickness and apply it to a highly efficient hydroelectric rim member. In this case, since the yield strength by the tensile test in the direction parallel to the rolling direction is important, the yield strength in the rolling direction is defined. The yield strength of the present invention is particularly suitable for steel plates up to 700 MPa.

The magnetic flux density B ₅₀ is more than 1.4 T, hydropower when the magnetic flux density B ₁₀₀ is 1.5T or higher magnetic flux density B ₅₀ is more than 1.4 T, the magnetic flux density B ₁₀₀ is equal to or greater than 1.5T, used for the rim member hydroelectric Can be made highly efficient.

Most of the rim members for hydroelectric power generation are joined by welding with the minimum value of the Vickers hardness of the weld heat affected zone being (average value of Vickers hardness of the base material −30) or more. By setting the Vickers hardness of the weld heat affected zone to (average value of Vickers hardness of base material −30) or more, it becomes possible to suppress defects and defects in the weld zone. The welding conditions at this time may be equivalent to the conditions described in the examples.

Next, the manufacturing method of the hot rolled steel sheet for magnetic poles of this invention is demonstrated.
The hot-rolled steel sheet for magnetic poles of the present invention heats a steel material (steel slab) having the above-described composition at 1100 ° C. or higher and 1350 ° C. or lower, and then completes rough rolling at a temperature of 1100 ° C. or higher and finish rolling temperature 840 ° C. It can be manufactured by performing hot rolling as described above, cooling at an average cooling rate of 30 ° C./s or higher within 3 seconds after finishing rolling, and then winding at 550 ° C. or higher and 700 ° C. or lower.

  In the present invention, the method for melting steel is not particularly limited, and a known melting method such as a converter or an electric furnace can be employed. In addition, secondary refining may be performed in a vacuum degassing furnace. After that, it is preferable to use a continuous casting method to form a slab (steel material) from the viewpoint of productivity and quality, but known casting methods such as ingot casting and blooming, thin slab continuous casting, etc. It is good also as a slab by the method.

Heating temperature of steel material: 1100 ° C or higher and 1350 ° C or lower Prior to hot rolling, it is necessary to heat the steel material to obtain a substantially homogeneous austenite phase. When the heating temperature is below 1100 ° C., coarse carbides containing Nb and Ti cannot be dissolved, and the strength and magnetic flux density are reduced. On the other hand, when the heating temperature exceeds 1350 ° C., the amount of scale generation increases, the scale bites during 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 higher and 1350 ° C or lower. Preferably they are 1150 degreeC or more and 1300 degrees C or less. However, when hot rolling the steel material, if the steel material after casting is in the temperature range of 1100 ° C or higher and 1350 ° C or lower, or if the carbide of the steel material is dissolved, the steel material is heated. Direct rolling may be performed without any problem.

Hot rolling to complete rough rolling at a temperature of 1100 ° C or higher and finish rolling temperature of 840 ° C or higher
C that does not bind to Nb, V, and Ti precipitates as Fe carbide. In order to precipitate almost all of the contained C as fine carbides containing Nb, V and Ti, it is necessary to complete rough rolling before finish rolling at 1100 ° C. or higher. When rough rolling is completed at less than 1100 ° C, carbides containing Nb, V, and Ti are coarsely precipitated in austenite by holding the strain introduced in rough rolling as the driving force for a long time, which is the strength and magnetic flux density. This is because the adverse effect on When the finish rolling temperature is less than 840 ° C., the ferrite transformation starts during finish rolling, resulting in a structure in which ferrite grains are extended. A large amount of dislocations are introduced into the extended ferrite grains, which causes a decrease in magnetic flux density. Accordingly, the finish rolling temperature is 840 ° C. or higher. Preferably it is 860 ° C or more. Although the temperature in finish rolling is less than 1100 ° C., tandem rolling in finish rolling has no time for precipitation and growth as compared with rough rolling, and thus the above-described adverse effect during rough rolling does not become obvious.

Cooling at an average cooling rate of 30 ° C / s or more within 3 seconds after finish rolling
The carbide containing Nb becomes finer as the transformation temperature from austenite to ferrite decreases. In order to obtain a carbide having an average particle size of 6 nm or less, the transformation temperature from austenite to ferrite needs to be 700 ° C. or less. For that purpose, it is necessary to cool at an average cooling rate of 30 ° C./s or more within 3 seconds after finishing rolling. The average cooling rate is an average cooling rate from the finish rolling temperature to 700 ° C.

Winding temperature: 550 ° C. or higher and 700 ° C. or lower If the winding temperature exceeds 700 ° C., the carbide is coarsened, and desired strength and magnetic properties cannot be obtained. On the other hand, when the temperature is lower than 550 ° C., the bainite phase is generated, so that the magnetic properties are degraded. From the above, the coiling temperature range is set to 550 ° C or more and 700 ° C or less. Preferably, they are 580 degreeC or more and 680 degrees C or less.

  Thus, the hot-rolled steel sheet for magnetic poles of the present invention is manufactured. Note that even if the hot-rolled steel sheet for magnetic poles of the present invention is passed through a continuous hot dipping line having an annealing temperature of 720 ° C. or lower, the material is not affected. Therefore, it is possible to further plate the steel plate surface and have a plating layer on the steel plate surface. In addition, since the material is not affected by the plating treatment or the composition of the plating bath, any of hot dip galvanizing treatment, alloying hot dip galvanizing treatment, and electrogalvanizing treatment can be applied as the plating treatment. Moreover, as a composition of a plating bath, what is necessary is just to include the 1 type (s) or 2 or more types of Zn, Si, Al, Ni, and Mg. That is, the composition of the plating layer formed in the plating process can include one or more of Zn, Si, Al, Ni, and Mg.

  The hot-rolled steel sheet for magnetic poles of the present invention obtained as described above is suitable for parts that require high magnetic poles, 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 into a predetermined shape by means such as shearing, punching, or laser cutting, laminated, and used as an electromagnetic member for a rim or core (such as a pole core). 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 laminating steel plates, it is preferable to electrically insulate between the steel plates to be laminated, such as by applying an insulating coating to the steel plates or sandwiching an insulating material therebetween.

Hot rolled steel sheets were manufactured under the hot rolling conditions shown in Table 2 for a steel material having a thickness of 250 mm having the composition shown in Table 1. Some of them were further subjected to an alloying hot dip galvanizing treatment. Alloying hot dipping treatment is a continuous hot dipping line where the annealing temperature is 700 ° C or less, the composition of the molten bath is Zn-0.13mass% Al, the temperature of the plating bath is 460 ° C, and the alloying temperature is 530 ° C. The coating weight was 45 to 65 g / m ² per side.

  Test pieces were collected from the hot-rolled steel sheet or alloyed hot-dip steel sheet having a thickness of 1.6 mm to 3.2 mm obtained as described above, and the structure was observed by the following method to evaluate the performance.

(I) Structure observation The area ratio of each phase was evaluated by the following method. Cut from hot-rolled steel sheet or galvannealed steel sheet so that the cross section parallel to the rolling direction becomes the observation surface, and the metal structure at the center of the plate thickness is etched with 3% nital, scanning type Ten fields of view were taken with an optical microscope at 400x magnification. The ferrite phase is a structure having a form in which corrosion marks and cementite are not observed in the grains. The bainite phase other than the ferrite phase, martensite phase, pearlite, and the like were separated from each other by image analysis, and the area ratio with respect to the observation field was obtained. In determining the area, the ferrite grain boundaries were counted as part of the ferrite phase.

  The average particle diameter of the carbide containing Nb was observed at 135,000 times or more using a transmission electron microscope, and the average particle diameter of 100 or more carbides was taken, and the equivalent circle diameter was determined as the particle diameter of each carbide. .

Fe precipitated amount, 10% AA electrolytic solution (10 vol% acetylacetone -1 wt% tetramethylammonium chloride - methanol) in, after constant current electrolysis about 0.2g at a current density of 20 mA / cm ^2, filtered from the electrolyte The amount of Fe contained in the precipitate was obtained by ICP-MS method, and the ratio to the mass of the ground iron electrolyzed by constant current electrolysis was obtained.

  The Nb precipitation amount (Nb precipitation ratio) was subjected to constant current electrolysis in the same manner as the Fe precipitation amount measurement method, and the Nb amount contained in the electrolytic solution was measured by the ICP-MS method. The amount of Nb contained in this electrolytic solution is the amount of Nb in a solid solution state, and the amount of Nb precipitation was determined by subtracting the amount of Nb in a solid solution state from the Nb content.

(Ii) Tensile test A JIS No. 5 tensile test piece was produced in parallel to the rolling direction from a hot-rolled steel sheet or an alloyed hot-dip steel sheet, and five tensile tests in accordance with the provisions of JIS Z 2241 (2011) were performed. Average yield strength (YS), tensile strength (TS), and total elongation (El) were determined. The crosshead speed in the tensile test was 10 mm / min.

(Iii) Magnetic flux density measurement A 30mm x 280mm sample is taken from a hot-rolled steel sheet or alloyed hot-dip steel sheet, and measured using a DC magnetic property measuring device in accordance with JIS C 2555, the magnetic flux density B ₅₀ and magnetic flux density B Asked for ₁₀₀ . B ₅₀ and B ₁₀₀ indicate magnetic flux densities at magnetization forces of 5000 A / m and 10000 A / m, respectively.

(Iv) Weldability evaluation As a welding test, carbon dioxide arc welding using a 1.2 mm diameter wire was performed and evaluated. The welding conditions were 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 (gap) of 1 mm. After welding, a cross section of the bead portion was cut out, and a Vickers hardness test with a test load of 0.49 N was performed in a direction crossing the welded portion at 0.5 mm intervals in the central portion of the thickness of the cross section. On the other hand, the hardness of the base material was an average value measured at five points with a test load of 0.49 N at a position 30 mm or more away from the weld. Table 3 shows the difference between the hardness of the base material (average value of the base material hardness) and the minimum hardness in the heat affected zone (the minimum value of the weld heat affected zone hardness).

  The results obtained as described above are shown in Table 3.

In all of the inventive examples, the yield strength YS in the rolling direction is 500 MPa or more, and the minimum value of the Vickers hardness of the heat affected zone is (average Vickers hardness of base material −30) or more. It can be seen that a hot-rolled steel sheet (alloyed hot-dip galvanized steel sheet) having excellent magnetic properties with a magnetic flux density B ₅₀ of 1.4 T or higher and a magnetic flux density B ₁₀₀ of 1.5 T or higher is obtained. On the other hand, a comparative example that is outside the scope of the present invention is inferior in yield strength, weldability, or magnetic properties.

Claims (8)

Component composition is mass%, C: 0.02% to 0.12%, Si: 0.1% to 0.7%, Mn: 0.8% to 1.6%, P: 0.03% or less, S: 0.005% or less, Al: 0.08 % Or less, N: 0.006% or less, Nb: 0.06% or more and 0.20% or less, with the balance being Fe and inevitable impurities,
The structure is that the ferrite phase is 98% or more in area ratio,
The precipitated Fe is 0.22% by mass or less with respect to the amount of Fe contained in the steel, the precipitated Nb is at least 80% by mass with respect to the amount of Nb contained in the steel, and the average particle size of the carbide containing the precipitated Nb is 6 nm. And
Yield strength in the rolling direction is 500 MPa or more, magnetic flux density B ₅₀ is 1.4 T or more, magnetic flux density B ₁₀₀ is 1.5 T or more, and the minimum value of Vickers hardness in the heat affected zone is the average value of Vickers hardness of the base metal −30) A hot-rolled steel sheet for magnetic poles, characterized in that it is above. Furthermore, the following formula (1) is satisfied, The hot rolled steel sheet for magnetic poles according to claim 1 characterized by things.

  3. The magnetic pole heat according to claim 1, wherein, in addition to the component composition, one or more of V: 0.01% or more and less than 0.05% and Ti: 0.01% or more and less than 0.05% are contained by mass%. Rolled steel sheet. The method for producing a hot-rolled steel sheet for magnetic poles according to any one of claims 1 to 3, wherein the steel material is heated at a temperature of 1100 ° C or higher and 1350 ° C or lower, and then roughened at a temperature of 1100 ° C or higher. After completion of rolling, perform hot rolling at a finish rolling temperature of 840 ° C or higher, cool it at an average cooling rate of 30 ° C / s or higher within 3 seconds after finishing rolling, and then wind at a temperature of 550 ° C or higher and 700 ° C or lower A method for manufacturing a hot-rolled steel sheet for magnetic poles.   The method for producing a hot-rolled steel sheet for magnetic poles according to claim 4, wherein the surface of the steel sheet is further plated.   6. The method for producing a hot-rolled steel sheet for magnetic poles according to claim 5, wherein the plating treatment is any one of a hot dip galvanizing treatment, an alloying hot dip galvanizing treatment, and an electrogalvanizing treatment.   The composition of the plating layer formed in the plating treatment includes one or more of Zn, Si, Al, Ni, and Mg. Production method.   A rim member for hydroelectric power generation comprising the hot-rolled steel sheet for magnetic poles according to any one of claims 1 to 3.