JP5245977B2 - Method for producing non-oriented electrical steel sheet - Google Patents

Description

  The present invention relates to a method for producing a non-oriented electrical steel sheet suitable as a material for a rotor core of a motor rotating at high speed. In particular, the present invention can withstand stress during rotation or stress variation during acceleration / deceleration, and requires excellent strength characteristics and magnetic characteristics, such as an embedded magnet motor (IPM motor) and salient pole type surface magnet motor (saliency pole). The present invention relates to a method for producing a non-oriented electrical steel sheet suitable as a material for a rotor core of a type SRM motor.

  In order to reduce greenhouse gases, new products that consume less energy are required in the fields of automobiles and home appliances. For example, in the automobile field, in order to reduce fuel consumption, there are a hybrid drive vehicle (HEV) of a gasoline engine and a motor or an electric vehicle driven by a motor. In the home appliance field, there are high-efficiency air conditioners and refrigerators with low annual electricity consumption. These common technologies are motors, and high efficiency of the motors is an important technology. In the process of increasing motor efficiency, motor drive systems have become more sophisticated and various rotational drive controls have become possible. That is, the number of motors capable of variable speed operation and high speed operation at commercial frequency or higher has been increased by frequency control of the drive power source.

  In order to realize such a high-speed rotating machine, it is necessary to develop a rotor that can withstand high-speed rotation. In general, the centrifugal force acting on the rotor is proportional to the rotational radius and proportional to the square of the rotational speed. For this reason, when operating at high speed rotation, the force acting on the rotor may exceed 500 MPa, for example. Therefore, a material with high yield strength is required for the rotor. Furthermore, during rotor high-speed rotation operation, repeated stresses such as external vibration and frequent acceleration / deceleration may occur, so the rotor material must not only have high yield strength but also high fatigue strength. Is also needed. Since it is most effective to increase the tensile strength as a means for increasing the fatigue strength, it can be said that a high yield strength and a high tensile strength are necessary for the material of the rotor rotating at high speed.

  Usually, a laminated non-oriented electrical steel sheet is used for the motor rotor, but the motor that rotates at high speed as described above may not satisfy the required strength. In that case, high strength cast steel or the like is used as a rotor material. However, since the motor rotor uses magnetic properties at the time of rotation, the material is required to have excellent mechanical properties as well as mechanical properties as described above. In other words, an integral cast steel rotor has a problem that the eddy current loss becomes very large and the efficiency of the motor decreases. Further, in the case of an IPM motor, there arises a problem that magnet characteristics deteriorate due to heat generated by loss in the rotor.

  As described above, the rotor core material of the motor rotating at high speed as described above must have high mechanical strength and magnetically have high frequency and low iron loss. As means for increasing the strength of steel sheets, in the field of cold-rolled steel sheets, methods such as solid solution strengthening, precipitation strengthening, grain refinement strengthening, transformation strengthening are generally used, but excellent magnetic properties such as high strength and high frequency low iron loss are used. In general, there is a contradictory relationship, and it has been extremely difficult to satisfy these simultaneously.

  In order to solve such a problem, several proposals have recently been made regarding non-oriented electrical steel sheets having high tensile strength. For example, Patent Document 1 proposes a method in which the Si content is increased to 3.5 to 7.0%, an element having a large solid solution hardening is added thereto, and the tensile strength is increased. In Patent Document 2, a normal non-oriented electrical steel sheet contains 2.0% or more and less than 4.0% Si, and at the same time, one or two of Nb and Zr, or one or two of Ti and V, or 2 A method has been proposed for producing non-oriented electrical steel sheets with high yield strength that combine mechanical and magnetic properties by utilizing various carbonitrides and also controlling hot rolling conditions and finish annealing conditions. Yes. Patent Document 3 proposes a method of increasing the tensile strength by including a metal phase made of Cu having a diameter of 1.0 μm or less in the steel material. Patent Document 4 proposes a method for increasing the tensile strength by dispersing a fine Cu precipitate phase, and Patent Document 5 and Patent Document 6 by dispersing a Cu precipitate phase and a carbonitride phase containing Ti and Nb. Has been.

JP 60-238421 A JP-A-6-330255 JP 2004-84053 A JP 2004-3000535 A JP 2005-344156 A JP 2007-31754 A

However, since the steel sheet obtained by the invention described in Patent Document 1 is very brittle, there is a problem that it is easily broken during cold rolling and the yield is very low.
In the invention described in Patent Document 2, since the finish annealing temperature is low, there is a problem that the crystal grain size of the steel sheet is very small and the iron loss is very inferior.
In the inventions described in Patent Documents 3 to 6, the above problem is solved, but surface defects peculiar to Cu-containing steel occur, and the yield of the product may be reduced.

  The present invention has been made in view of the above problems, and can increase the strength by the fine precipitates of Cu, has low iron loss at high frequencies, and suppresses surface defects peculiar to Cu-containing steel. The main object is to provide a method for producing a grain-oriented electrical steel sheet.

  As a result of intensive research to suppress surface defects peculiar to Cu-containing steels, the present inventors have obtained excellent strength characteristics and magnetic properties by optimizing the slab heating conditions when the slab is subjected to hot rolling. The present inventors have found that a non-oriented electrical steel sheet having characteristics and further excellent surface properties can be obtained, thereby completing the present invention.

That is, the present invention is, in mass%, C: 0.02% or less, Si: 1% or less, Mn: 1% or less, P: 0.2% or less, S: 0.03% or less, Al: 2% 4% or less, Ni: 2% or less, Cu: more than 1%, 3% or less, and at least one element selected from the group consisting of Ti, Nb, V, and Zr in total is 0.001% or more and 0.0. A method for producing a non-oriented electrical steel sheet comprising hot rolling, cold rolling and finish annealing on a slab having a steel composition comprising Fe and impurities in the balance, the content of which is within 1% or less, Provided is a method for producing a non-oriented electrical steel sheet characterized by heating to 1120 ° C. or higher and 1240 ° C. or lower at a rate of temperature increase of 400 ° C./h or higher when subjected to hot rolling at a temperature range of 800 ° C. or higher.
In addition, the temperature prescribed | regulated here is slab surface temperature.

  According to the present invention, a non-oriented electrical steel sheet having high strength and good magnetic properties and surface properties can be obtained by appropriately controlling the steel composition and production conditions of the steel sheet. By using such a method for producing a non-oriented electrical steel sheet according to the present invention, it is possible to provide a non-oriented electrical steel sheet suitable for a motor rotor that can be used stably without deformation or breakage during operation. It becomes.

  In the present invention, it is possible to produce a non-oriented electrical steel sheet having low iron loss at high frequencies and high strength, and further suppressing surface defects peculiar to Cu-containing steel. If an iron core manufactured using such a non-oriented electrical steel sheet is incorporated into a motor rotor that rotates at high speed, the motor efficiency will not only increase, but it can be used stably for a long time without being deformed or broken during operation. It becomes. Such energy-saving effects can contribute to the creation of a future society with less impact on the global environment.

It is a surface external appearance photograph of the hot-rolled steel plate after the pickling of the trial numbers 5, 6, and 16 in an Example.

  Hereinafter, the manufacturing method of the non-oriented electrical steel sheet of this invention is demonstrated in detail.

The manufacturing method of the non-oriented electrical steel sheet of the present invention is, in mass%, C: 0.02% or less, Si: 1% or less, Mn: 1% or less, P: 0.2% or less, S: 0.03. %, Al: 2% or more, 4% or less, Ni: 2% or less, Cu: more than 1%, 3% or less, and at least one element selected from the group consisting of Ti, Nb, V and Zr in total A method for producing a non-oriented electrical steel sheet comprising hot rolling, cold rolling and finish annealing on a slab having a steel composition comprising 0.001% or more and 0.1% or less, the balance being Fe and impurities. And when using the said slab for hot rolling, it heats to 1120 degreeC or more and 1240 degrees C or less by making the temperature increase rate in a temperature range of 800 degreeC or more into 400 degrees C / h or more.
“%” Indicating the content of each element means “mass%” unless otherwise specified.

According to the present invention, by properly controlling the steel composition and production conditions of the steel sheet, particularly by properly controlling the slab heating conditions when the slab is subjected to hot rolling, good strength characteristics and magnetic characteristics can be obtained. In addition, a non-oriented electrical steel sheet having even better surface properties can be obtained.
Hereinafter, the steel composition and each process of the slab in the manufacturing method of the non-oriented electrical steel sheet of this invention are demonstrated.

1. Steel composition (1) C
C is an element effective for increasing the strength of the steel sheet. However, if the C content exceeds 0.02%, carbides such as cementite and ε-carbide precipitate, and the magnetic property deterioration may become remarkable. Therefore, the C content is 0.02% or less. In order to further improve the magnetic properties, particularly to improve the iron loss, the upper limit of the C content is preferably 0.005%. On the other hand, when the carbide strengthening element such as Ti, Nb, V, Zr or the like is contained in an amount of 0.01% or more to enhance precipitation strengthening, the C content is preferably controlled to 0.005% to 0.02%. .

(2) Si
Si is an effective element for increasing the strength of the steel sheet and improving the magnetic properties. However, in the steel sheet that must contain Cu as in the present invention, Cu melt embrittlement cracking is caused by hot rolling. This may cause the surface properties to deteriorate. If the Si content exceeds 1%, the surface properties of the hot-rolled steel sheet may deteriorate and the product yield may decrease. Therefore, the Si content is 1% or less. Furthermore, in order to suppress surface defects, it is preferable that the Si content is 0.2% or less. Si is an element contained as an unavoidable impurity, but the lower the content, the better. Therefore, there is no need to specifically limit the lower limit.

(3) Mn
Mn is an unavoidable impurity and need not be added. However, since Mn increases the specific resistance of steel and is effective in reducing iron loss, it may be positively added. In order to acquire the effect, it is preferable to make it contain 0.1% or more. On the other hand, if the Mn content exceeds 1%, the raw material cost may increase. Therefore, the Mn content is limited to 1% or less.

(4) P
P is an unavoidable impurity and need not be added. However, since P is an element effective for increasing the strength of the steel sheet by solid solution strengthening, P may be contained. In order to acquire the effect, it is preferable to make it contain 0.03% or more. On the other hand, if the P content exceeds 0.2%, the toughness of the steel sheet deteriorates and the hot-rolled steel sheet may be broken. Therefore, the P content is limited to 0.2% or less.

(5) S
S is an unavoidable impurity and does not need to be added. If the S content exceeds 0.03%, coarse Mn and Cu-containing sulfides are formed, the toughness of the steel deteriorates, and there is a risk of fracture during cold rolling. Therefore, the S content is limited to 0.03% or less. In order to improve magnetic properties, the S content is preferably 0.006% or less.

(6) Al
Al increases the specific resistance of steel and is effective in reducing iron loss. In the present invention, since the Si content having the same effect is reduced, it is necessary to positively add Al. However, if the Al content exceeds 4%, the saturation magnetic flux density is remarkably lowered, and the core performance may be deteriorated. On the other hand, it is necessary to contain 2% or more of Al in order to reduce high-frequency iron loss. Therefore, the Al content is limited to 2% or more and 4% or less. More preferably, it is 2.5% or more and 3.5% or less.

(7) Ni
Ni is effective in preventing surface defects of the steel sheet. However, in the present invention, since surface defects are suppressed by managing the slab heating conditions, it is not necessary to contain a considerable amount of Ni as in the prior art. On the other hand, Ni is effective in increasing the strength of the steel sheet by solid solution strengthening, so Ni may be added. However, since Ni is an expensive element, its content is made 2% or less from the viewpoint of manufacturing cost.

(8) Cu
Cu is an essential element in the present invention. When the Cu precipitate is very fine, there is an effect of improving the strength characteristics without substantially degrading the magnetic characteristics. However, when the Cu content is 1% or less, there is a possibility that a sufficient strength increase due to Cu precipitation cannot be obtained. On the other hand, the age hardening amount increases as the Cu content increases. However, if it exceeds 3%, Cu precipitates are unevenly dispersed during finish annealing, and the strength after aging heat treatment is reduced. May decrease. Therefore, the Cu content is limited to more than 1% and 3% or less.

(9) Ti, Nb, V and Zr
Ti, Nb, V, and Zr have a function of improving the strength by forming precipitates such as carbides. In order to obtain the effect of the above action, the total content of Ti, Nb, V and Zr is set to 0.001% or more. On the other hand, if the total content of Ti, Nb, V and Zr exceeds 0.1%, the precipitates may become coarse and the magnetic properties may deteriorate. Therefore, the total content of Ti, Nb, V and Zr is set to 0.001% or more and 0.1% or less. Preferably, it is 0.01% or more and 0.1% or less.

  In the present invention, the steel composition has a total of at least one element selected from the group consisting of Ti, Nb, V and Zr in the range of 0.001% to 0.1%, preferably 0.01% to 0%. The content of any one element of Ti, Nb, V, or Zr is preferably 0.001% or more independently in order to surely improve the strength. More preferably, the content is 0.01% or more.

2. Hot rolling step In the present invention, when the slab having the above steel composition is subjected to hot rolling, it is heated to 1120 ° C or higher and 1240 ° C or lower at a heating rate of 800 ° C or higher at 400 ° C / h or higher. As described above, the temperature defined here is the slab surface temperature.
In the present invention, it is important that the conditions of the slab surface temperature and the temperature increase rate in the slab heat treatment when the slab is subjected to hot rolling are appropriate.
When the slab surface temperature when the slab is subjected to hot rolling is less than 1120 ° C., the oxide scale generated during slab heating becomes difficult to peel off, and surface flaws due to scale pressing tend to occur. On the other hand, when the slab surface temperature when the slab is subjected to hot rolling exceeds 1240 ° C., surface cracks are likely to occur due to embrittlement caused by molten Cu. Similarly, surface cracks are likely to occur when the rate of temperature increase in the temperature range of 800 ° C. or higher is less than 100 ° C./h. Therefore, when the slab is subjected to hot rolling, the heating rate in the temperature range of 800 ° C. or higher is set to 400 ° C./h or higher and heated to 1120 ° C. or higher and 1240 ° C. or lower.

  The reason why surface cracks occur due to embrittlement caused by molten Cu depending on slab heating conditions is presumed as follows. That is, when the rate of temperature increase of slab heating is slow, with the progress of oxidation of the slab surface layer, the molten Cu phase aggregates at the interface between the scale and steel and induces surface cracks during hot rolling, When the rate of temperature increase is high, the molten Cu phase is taken into the scale and dispersed in various points, and is hardly present at the interface between the scale and the steel. As a result, surface cracks during hot rolling are suppressed. It is guessed.

  In addition, since it is so preferable that the temperature increase rate in a temperature range of 800 degreeC or more is quick, the upper limit of temperature increase rate does not need to be prescribed | regulated in particular. From the viewpoint of homogenizing the temperature distribution in the slab, it is preferable that the rate of temperature rise in the temperature range of 800 ° C. or higher is 800 ° C./h or lower.

  Further, since the scale hardly grows in the temperature range below 800 ° C., the rate of temperature rise in the temperature range does not affect the embrittlement caused by the molten Cu. Therefore, it is not necessary to specify the rate of temperature rise in the temperature range below 800 ° C.

  The water vapor concentration in the heating furnace atmosphere for heating the slab is preferably 10% or more and 30% or less.

  A general method can be used as hot rolling after slab heating. Conditions such as the finishing temperature and coiling temperature in hot rolling are appropriately selected depending on the steel composition of the slab, the thickness of the target steel sheet, and the like. In addition, in order to improve the toughness of a hot-rolled steel plate, it is preferable that winding temperature shall be 550 degreeC or more.

  A hot-rolled steel sheet is usually subjected to cold rolling after removing scales generated on the surface of the steel sheet during hot rolling by pickling.

3. Cold Rolling Process In the present invention, the hot rolled steel sheet obtained by the hot rolling process is cold rolled to obtain a cold rolled steel sheet.

This step may be a step of performing cold rolling two or more times with intermediate annealing on the hot-rolled steel sheet. Although the intermediate annealing is not necessarily essential, the intermediate annealing has the advantage that the ductility of the steel sheet is improved and the breakage in cold rolling is reduced.
The conditions such as the annealing temperature in the intermediate annealing are preferably the same as those in hot-rolled sheet annealing described later.

4). Finish annealing step In the present invention, the finish annealing is performed on the cold rolled steel sheet obtained by the cold rolling step.

In the present invention, the strength can be increased by performing an aging heat treatment after finish annealing, but in order to further improve the strength and magnetic properties after the aging heat treatment, it is preferable to appropriately control the finish annealing conditions. .
The finish annealing temperature is preferably 900 ° C. or higher and 1100 ° C. or lower. If the finish annealing temperature is less than the above range, recrystallization grain growth is insufficient and the magnetic properties may be significantly deteriorated. On the other hand, when the finish annealing temperature exceeds the above range, the particle size of the steel sheet becomes extremely coarse, Cu precipitates after the aging heat treatment are dispersed unevenly, and the strength may be lowered. In order to further reduce the iron loss, the higher the finish annealing temperature, the better. It is more preferable to set the temperature to 950 ° C. or higher.

5. Hot-rolled sheet annealing step In the present invention, the hot-rolled sheet annealing step is not essential, but may be employed as appropriate because it is a useful process for improving the efficiency of cold rolling. When hot-rolled sheet annealing is performed, it is preferable that hot-rolled sheet steel having the above-described steel composition is subjected to hot-rolled sheet annealing that is held at a temperature of 600 ° C. or higher and 900 ° C. or lower for 10 seconds or longer. Pickling in the case of performing hot-rolled sheet annealing may be performed either before or after hot-rolled sheet annealing.

  The annealing temperature in the hot-rolled sheet annealing is preferably 600 ° C. or higher and 900 ° C. or lower. If the annealing temperature is less than the above range, the strength of the steel sheet may be increased and cold rolling may be difficult. On the other hand, even if the annealing temperature exceeds the above range, solid solution / reprecipitation of Cu occurs, the strength of the steel sheet increases, and cold rolling may become difficult. A more preferable annealing temperature is 650 ° C. or higher and 850 ° C. or lower.

  The holding time at the annealing temperature is preferably 10 seconds or longer. When holding time is less than the said range, the intensity | strength of a hot-rolled steel plate becomes high and cold rolling may become difficult. The holding time is more preferably 2 hours or more. On the other hand, the upper limit of the holding time is not particularly limited, but is preferably 48 hours or less from the viewpoint of economy.

6). Other Steps In the present invention, after the finish annealing step, according to a general method, a coating step of applying an insulating film made of only an organic component, only an inorganic component, or an organic-inorganic composite to a steel sheet surface may be performed. Good. Further, the coating process may be a process of applying an insulating coating that exhibits adhesive ability by heating and pressurizing. As a coating material exhibiting adhesive ability, an acrylic resin, a phenol resin, an epoxy resin, a melamine resin, or the like can be used.

  Moreover, in this invention, a high-strength non-oriented electrical steel sheet can be manufactured by performing an aging heat treatment after finish annealing.

  The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and the present invention has any configuration that has substantially the same configuration as the technical idea described in the claims of the present invention and that exhibits the same effects. Are included in the technical scope.

Hereinafter, the present invention will be described specifically by way of examples.
A slab having a steel composition shown in Table 1 (size: thickness 30 mm, width 70 mm, length 100 mm) is heated under the slab heating conditions shown in Table 2, and descaling with high-pressure water is performed, followed by a thickness of 2.0 mm. The steel sheet was hot-rolled and gradually cooled from 600 ° C. to obtain a hot-rolled steel sheet. The obtained hot-rolled steel sheet was subjected to temper rolling with an elongation of 1%, descaled by pickling, and the surface of the steel sheet was visually observed to check for wrinkles. Table 2 below shows the evaluation results of the surface properties.

Moreover, the said steel plate was cold-rolled to thickness 0.35mm, and the obtained cold-rolled steel plate was finish-annealed at 1000 degreeC.
A 55 mm square single-plate magnetic test piece was collected from the steel plate thus obtained and subjected to an aging heat treatment at 480 ° C. for 2 hours, and then the iron loss W _10/400 and the magnetic flux density B ₅₀ were measured. Their magnetic properties were average values in the rolling direction and the direction perpendicular thereto. Furthermore, a tensile test piece of JIS No. 13B was taken from the steel plate before aging heat treatment in the rolling direction, and after the same aging heat treatment as described above, the specified tensile test was performed on JIS-Z-2241 to measure the yield strength YS and tensile strength TS. . As a result, the iron loss of all the steel plates tested was 20 to 25 W / kg, the magnetic flux density was 1.57 to 1.60 T, the yield strength was 680 to 770 MPa, and the tensile strength was in the range of 780 to 870 MPa. Excellent magnetic properties And strength characteristics were confirmed.

Run No. 2, 3 Contact and 17 to 21 satisfying the steel composition and the slab heating conditions specified in the present invention, it is of course excellent in strength and magnetic properties, surface properties of the steel sheet were good. On the other hand, the trial numbers 1, 4 to 6 and 9 to 12 which do not satisfy the slab heating conditions defined in the present invention, and the trial numbers 15 and 16 which do not satisfy the steel composition defined in the present invention are excellent in strength and magnetic properties. Many surface flaws occurred.
FIGS. 1A to 1C show the occurrence of surface defects in the hot-rolled steel sheet after pickling of trial numbers 5, 6 and 16, respectively. In Test No. 5, surface defects due to embrittlement due to molten Cu were observed. In Test Nos. 6 and 16, surface defects due to scale indentation were observed. Thus, when it manufactured without combining steel composition and slab heating conditions appropriately, the surface property of the steel plate deteriorated remarkably.

Claims (1)

In mass%, C: 0.02% or less, Si: 1% or less, Mn: 1% or less, P: 0.2% or less, S: 0.03% or less, Al: 2% or more and 4% or less, Ni : 2% or less, Cu: more than 1%, 3% or less, and at least one element selected from the group consisting of Ti, Nb, V and Zr within the range of 0.001% or more and 0.1% or less in total A non-oriented electrical steel sheet that is hot-rolled, cold-rolled, and finish-annealed into a slab having a steel composition consisting of Fe and impurities.
A method for producing a non-oriented electrical steel sheet, characterized in that when the slab is subjected to hot rolling, the heating rate in a temperature range of 800 ° C. or higher is 400 ° C./h or higher to 1120 ° C. or higher and 1240 ° C. or lower.