JP2015212403A - Method for manufacturing nonoriented electromagnetic steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a nonoriented electromagnetic steel sheet capable of hot rolling a low ally-based nonoriented electromagnetic steel sheet having γ→α transformation with high productivity without reducing magnetic properties.SOLUTION: In a method for a nonoriented electromagnetic steel sheet including re-heating, then hot rolling including rough rolling and finishing rolling, cold rolling and finishing rolling a slab having a component composition containing, by mass%, C:0.005% or less, Si:0.05 to 1.0%, Mn:1% or less, Al:1.0% or less, P:0.2% or less, S:0.005% or less and N:0.005% or less and the balance Fe with inevitable impurities and having γ-α transformation, total rolling reduction rate in the finishing rolling is 93% or more, rolling finishing temperature of Artransformation point or less and the finishing rolling is conducted at maximum rolling speed of a finishing rolling device.

Description

  TECHNICAL FIELD The present invention relates to a method for manufacturing a non-oriented electrical steel sheet, and more specifically, to a method for manufacturing a non-oriented electrical steel sheet that can increase productivity in hot rolling.

  Non-oriented electrical steel sheets are widely used as iron core materials for motors and transformers of electrical equipment. This non-oriented electrical steel sheet is strongly required to have a high magnetic flux density and a low iron loss against the background of global energy saving in recent years.

  One method for increasing the magnetic flux density of a non-oriented electrical steel sheet is to improve the texture by subjecting the steel sheet after hot rolling to hot rolling. That is, by annealing the hot-rolled sheet and coarsening the grain size before cold rolling, the formation of {110} recrystallized grains advantageous for magnetic properties in recrystallization annealing after cold rolling This is because it can promote and suppress the formation of {111} recrystallized grains that are disadvantageous to magnetic properties. However, performing hot-rolled sheet annealing causes an increase in manufacturing cost. For this reason, low-alloy non-oriented electrical steel sheets (hereinafter also referred to as “lower steel”) are required to omit hot-rolled sheet annealing in order to reduce production costs.

  Such a low-grade steel component steel material generally undergoes a γ → α transformation when the temperature is lowered from a high temperature to a low temperature. In the finish rolling of hot rolling, from the γ phase to the α phase. Rolled while undergoing phase transformation. Therefore, in order to increase the grain size after hot rolling and improve the magnetic flux density, in the above finish rolling, the crystal grains are controlled by rolling so as to promote the transformation from the γ phase to the α phase. It can be said that it is preferable to coarsen.

Therefore, for example, in Patent Document 1, by controlling the reduction ratio of finish rolling in hot rolling, the finish rolling finish temperature, etc., the crystal structure before cold rolling, that is, the crystal grains after hot rolling are coarsened. Technologies for increasing the magnetic flux density have been proposed.
In addition, Patent Document 2 discloses that the composition of the slab of the material is optimized and the temperature conditions in the hot rolling and finish annealing are optimized, so that the grains in the strain relief annealing can be performed without performing hot rolling sheet annealing. Technologies for improving growth have been proposed.

Japanese Patent Laid-Open No. 11-172333 JP 2007-217744 A

However, in the above prior art, in order to promote the γ → α transformation during hot rolling, the hot rolling finish temperature is set to the γ → α transformation completion temperature (Ar ₁₎ after increasing the rolling reduction ratio of finish rolling. Although the transformation point is set below, in order to lower the hot rolling end temperature, it is necessary to reduce the finish rolling speed, which has been a factor that hinders productivity.

  The present invention has been made in view of the above-mentioned problems of the prior art, and the purpose thereof is to produce a low alloy non-oriented electrical steel sheet having a γ → α transformation without deteriorating magnetic properties. The object is to propose a method for producing a non-oriented electrical steel sheet that can be hot-rolled with high productivity.

In order to solve the above-mentioned problems, the inventors have made extensive studies by paying attention to the relationship between the cooling rate and the reduction pattern in hot rolling.
As a result, in order to increase the rollability in hot rolling, first, the rolling speed in finish rolling is set to a desired high rolling speed, and in order to realize this, the above rolling speed in finish rolling is achieved. When calculating the lower limit value of the average cooling rate in the finishing mill necessary for the above, then calculating the maximum sheet bar thickness at which the lower limit value of the average cooling rate is obtained, and performing hot rolling based on the result We came up with a good idea and developed the present invention.

That is, the present invention is C: 0.005 mass% or less, Si: 0.05 to 1.0 mass%, Mn: 1 mass% or less, Al: 1.0 mass% or less, P: 0.2 mass% or less, S: 0 0.005 mass% or less and N: 0.005 mass% or less, the remainder having a component composition consisting of Fe and inevitable impurities, and the component composition is reheated to a slab having a γ-α transformation, In a method for producing a non-oriented electrical steel sheet comprising hot rolling, cold rolling, and finish annealing comprising rough rolling and finish rolling, the total rolling reduction of the above finish rolling is 93% or more, and the rolling end temperature is Ar ₁ transformation point. The following is set, the rolling speed of the finish rolling is determined, the lower limit value of the average cooling rate in the finishing mill necessary for rolling at the rolling speed is calculated, and then the lower limit value or more The target sheet bar thickness is determined by calculating the maximum sheet bar thickness at which a uniform cooling rate can be obtained and the minimum sheet bar thickness that satisfies the above-mentioned total rolling reduction in finish rolling.Based on these results, in rough rolling, the slab is A method for producing a non-oriented electrical steel sheet is proposed in which the sheet bar is rolled to the target sheet bar thickness and the sheet bar is rolled while being cooled at the calculated cooling rate.

  The slab used in the method for producing a non-oriented electrical steel sheet of the present invention is selected from Sn: 0.005-0.1 mass% and Sb: 0.005-0.1 mass% in addition to the above component composition. It is characterized by containing 1 type or 2 types.

  According to the present invention, even in a low alloy non-oriented electrical steel sheet having a γ-α transformation, it becomes possible to manufacture a non-oriented electrical steel sheet having excellent magnetic flux density with high productivity, thereby reducing manufacturing costs. Greatly contributes to.

First, the component composition which the steel raw material (slab) used for manufacture of the non-oriented electrical steel sheet of the present invention should have is described.
C: 0.005 mass% or less When C is contained in the product steel plate in an amount exceeding 0.005 mass%, magnetic aging occurs and the iron loss characteristics deteriorate with time. Moreover, since C has an effect of lowering the Ar ₁ transformation point, it is desirable to reduce it as much as possible from the viewpoint of ensuring productivity. Then, the non-oriented steel plate of this invention makes C content 0.005 mass% or less.

Si: 0.05 to 1.0 mass%
Si is an element effective in reducing iron loss because it increases the specific resistance of steel and reduces eddy current loss. Si also has the effect of increasing the Ar ₁ transformation point. In order to obtain these effects, it is necessary to add at least 0.05 mass% of Si. On the other hand, if it exceeds 1.0 mass%, the magnetic flux density is lowered, or a defect called ridging is likely to occur. Therefore, Si is set to a range of 0.05 to 1.0 mass%. Preferably it is the range of 0.1-0.5 mass%.

Mn: 1 mass% or less Mn has an effect of lowering the Ar ₁ transformation point and is also a harmful element that lowers the saturation magnetic flux density. Since these adverse effects increase when the mass exceeds 1 mass%, the upper limit is set to 1 mass%. Preferably it is 0.5 mass% or less. However, Mn is an element that is more difficult to oxidize than Fe, and excessive removal in the steelmaking process is accompanied by an increase in cost. It also has the effect of improving hot workability. Therefore, it is preferable to contain 0.1 mass% or more from a viewpoint of manufacturing cost.

Al: 1.0 mass% or less Al is an element that forms a nitride and suppresses the coarsening of crystal grains when the content is very small, but during hot rolling when it contains a large amount In this cooling stage, there are effects of suppressing the fine precipitation of various nitrides and promoting grain growth in annealing, particularly strain relief annealing. The effects and to reduce the eddy current loss by increasing the specific resistance of the steel, so has the effect of increasing the Ar ₁ transformation point, may be added more than 0.2 mass%. However, if added over 1.0 mass%, disadvantages such as a decrease in magnetic flux density and an increase in ridging are likely to occur. Therefore, when it contains Al, it is 1.0 mass% or less. Preferably it is 0.6 mass% or less.

P: 0.2 mass% or less P is an element added for the purpose of increasing the hardness of the steel sheet and preventing the occurrence of “sag” and “burr” in the punching process. However, if added over 0.2 mass%, the steel becomes too hard and the workability itself decreases. Therefore, the content of P is set to 0.2 mass% or less. Preferably it is the range of 0.05-0.15 mass%.

S: 0.005 mass% or less S is an element that forms a sulfide with Mn, Cu, or the like and precipitates, and hinders grain growth in annealing, particularly strain relief annealing, so it is desirable to reduce it as much as possible. In particular, when the content exceeds 0.005 mass%, this influence cannot be ignored. Therefore, S is set to 0.005 mass% or less. Preferably it is 0.001 mass% or less.

N: 0.005 mass% or less N is an element that forms a nitride with Al, Ti, V, or the like, precipitates, and hinders grain growth in annealing, particularly strain relief annealing, and is preferably reduced as much as possible. In particular, when the content exceeds 0.005 mass%, this influence cannot be ignored. Therefore, N is set to 0.005 mass% or less. Preferably it is 0.001 mass% or less.

The steel material used for this invention may contain 1 type or 2 types chosen from Sn and Sb other than the said component in 0.005-1.0 mass%, respectively.
Sn and Sb are effective in improving the texture and preventing oxidation, nitridation, and carburization during annealing. However, in both elements, the effect is small at 0.005 mass% or less, and the effect is saturated when it exceeds 1.0 mass%. Therefore, when adding, it is preferable to add in the said range.

  In the steel material used in the present invention, the balance other than the above components is Fe and inevitable impurities. However, as long as the effects of the present invention are not impaired, the inclusion of components other than those described above is not rejected.

The steel material used in the present invention is characterized by having an Ar ₁ transformation point at which a transformation from γ to α (γ → α transformation) occurs when the temperature falls during hot rolling due to the above component composition. Here, the Ar ₁ transformation point in the present invention is a region of C ≧ 0.02 mass%, not a temperature at which austenite transforms into ferrite and cementite (about 720 ° C.), but a region of C ≦ 0.02 mass%. This is the temperature at which austenite completes transformation to ferrite, and the lower the C, the higher the Ar ₁ transformation point and the closer to the A ₃ point (about 910 ° C.).

Next, the manufacturing method of the non-oriented electrical steel sheet of this invention is demonstrated.
In the method for producing a non-oriented electrical steel sheet according to the present invention, after reheating a steel material (slab) having the above-described component composition, hot rolling including rough rolling and finish rolling, cold rolling, and finishing are performed. This is a manufacturing method comprising a series of annealing steps, and there is no difference from the conventionally known methods for manufacturing non-oriented electrical steel sheets, except for the slab component composition and the hot rolling process.

  First, the steel material (slab) used for the production of the non-oriented electrical steel sheet of the present invention is a steel that satisfies the above component composition, which is melted by a generally known refining process using a converter, a vacuum degassing apparatus, etc. It is preferable to produce by the ingot-making-slab rolling method or the continuous casting method. Moreover, it is good also as a thin slab of 100 mm or less by a direct casting method.

Next, the slab is then reheated to a predetermined temperature and then subjected to hot rolling. The reheating method of the slab is not particularly limited, and a known method can be applied. Moreover, it is preferable that the reheating temperature of a slab shall be the range of 1000-1200 degreeC. If it is less than 1000 ° C., hot rolling becomes difficult. On the other hand, if it exceeds 1200 ° C., it is not preferable in terms of thermal energy cost.
In addition, when the hot rolling temperature mentioned later is securable, you may employ | adopt the direct rolling which carries out the hot rolling immediately, without reheating the continuously cast slab.

The subsequent hot rolling is the most important step in the present invention, and a generally known hot rolling method including rough rolling and finish rolling can be applied, but the following conditions must be satisfied.
First, in the finish rolling in the hot rolling of the present invention, the transformation from the γ phase to the α phase is completed during the finish rolling, and the rolling is finished in the α phase region, that is, the finish rolling finish temperature FDT is set to the Ar ₁ transformation point. It is necessary to: This is because when the FDT exceeds the Ar ₁ transformation point, the γ-α transformation occurs after the end of hot rolling, so that the crystal grains become finer and the magnetic properties deteriorate.

In the finish rolling in the present invention, the total rolling reduction needs to be 93% or more. If the total rolling reduction of the finish rolling is less than 93%, the driving force of the γ → α transformation is insufficient even if the FDT is below the Ar ₁ transformation point, and the γ → α transformation is not completed during the finish rolling. This is because the γ → α transformation occurs even after the completion of the process, so that the crystal grains become finer and the magnetic properties may be deteriorated.

Furthermore, in the hot rolling in the present invention, it is important to first set a desired rolling speed in the finish rolling in order to further improve the productivity of the hot rolling mill after satisfying the above conditions. .
Here, the said rolling speed means the steel plate speed | rate of a finishing rolling mill delivery side (final stand delivery side). This is because the productivity of the finishing mill is proportional to the rolling speed of the finishing mill.

And in this invention, in order to achieve the rolling speed of the finishing mill set to the above, hot rolling is implemented in the following procedures.
・ First, consider the maximum rolling speed in the finishing mill (steel speed at the end of finishing rolling at the end of finishing rolling) from the specifications and capabilities of the finishing mill and the steel type, dimensions, hot rolling temperature, etc. Then, an arbitrary rolling speed that is close to the maximum rolling speed is set.
-Next, based on the determined rolling speed, the lower limit value of the cooling rate of the steel sheet in the finishing mill is determined.
Next, the maximum sheet bar thickness capable of obtaining a cooling rate equal to or higher than the above lower limit value and the minimum sheet bar thickness required to satisfy the total rolling reduction (93% or more) in finish rolling are obtained, and the sheet satisfying both Determine the bar thickness. In addition, although the determination of the sheet bar thickness after the said rough rolling may be calculated | required by calculation, you may estimate from the relationship between the sheet bar thickness and the cooling rate in the past finishing rolling results accumulate | stored for every steel type.
Next, hot rolling is performed according to the above determination. That is, in rough rolling, the slab is rolled to the above-determined sheet bar thickness, and in finish rolling, finish rolling is finished in the α phase region, that is, the finish rolling finish temperature is set as the Ar ₁ transformation point. The steel plate is rolled to a predetermined plate thickness at the maximum rolling speed while the steel plate is cooled at the lower limit value or more of the cooling rate.

Subsequently, the hot-rolled steel sheet (hot-rolled sheet) is cold-rolled and finish-annealed, and an insulating film is formed as necessary to obtain a product plate.
For the cold rolling, a generally known method can be applied, and there is no particular limitation.
Moreover, although the finish annealing following the said cold rolling is not restrict | limited in particular, It is preferable to employ | adopt the continuous annealing hold | maintained at 650-800 degreeC for 2 to 60 seconds. If the soaking temperature is less than 650 ° C. or the soaking time is less than 2 seconds, the crystal grains do not grow sufficiently, and the good magnetic properties that are the main point of the present invention may not be obtained. On the other hand, if the soaking temperature exceeds 800 ° C. or the soaking time exceeds 60 seconds, the crystal grains are excessively coarsened and the punching processability may be deteriorated.
Further, the insulating film formed on the product plate is not particularly limited, and generally known ones can be used.

Melted in a converter and subjected to vacuum degassing, C: 0.003 mass%, Si: 0.12 mass%, Mn: 0.25 mass%, P: 0.08 mass%, S: 0.003 mass%, A steel containing Al: 0.001 mass% and N: 0.002 mass%, with the balance being composed of Fe and inevitable impurities, was continuously cast into a slab. In addition, from the component composition of the steel, the Ar ₁ transformation point is about 865 ° C.
Next, the slab was reheated to a temperature of 1100 ° C., then rough-rolled, hot-rolled to finish rolling at a rolling end temperature of 830 ° C. (lower than the Ar ₁ transformation temperature), and a plate thickness of 2.6 mm A hot-rolled sheet was used. Here, the rolling speed of the steel sheet was set to 950 m / min in consideration of the capability of the finishing mill used for the hot rolling.

The hot finish rolling was performed under the following four conditions.
Condition A: The lower limit of the cooling rate at the time of finish rolling necessary to achieve the rolling speed set above in the finishing mill is set to 85 ° C./s, and then the maximum at which a cooling rate equal to or higher than the lower limit is obtained. The sheet bar thickness, and the target sheet bar thickness in rough rolling from the minimum sheet bar thickness at which the total rolling reduction of finish rolling is 93% or more, are determined based on the past rolling performance of steel sheets having the same component composition, Based on the result, in rough rolling, the slab is rolled to the target sheet bar thickness, and in finish rolling, the target bar is cooled at the set rolling speed while the sheet bar is cooled at a value equal to or higher than the lower limit of the cooling speed. Roll to thickness (2.6 mm) (Invention example)
Condition B: Hot rolling without controlling the cooling rate and sheet bar thickness (Comparative Example 1)
-Condition C: In condition A, the finish rolling finish temperature is rolled at an Ar ₁ transformation point or higher (Comparative Example 2).
Condition D: In condition A, the thickness of the sheet bar after the rough rolling is changed, and the total rolling reduction in finish rolling is set to 90% for hot rolling (Comparative Example 3).
At this time, the time required for rolling from biting to bottom end in hot finish rolling and the steel plate speed on the exit side of the finish mill at the end of finish rolling were measured.

Subsequently, the hot-rolled sheet was cold-rolled to form a cold-rolled sheet having a thickness of 0.5 mm, and then the cold-rolled sheet was subjected to finish annealing at 700 ° C. for 30 seconds in a 10 vol% H ₂ -90 vol% N ₂ atmosphere. To give a product plate (coil). In addition, the said product board manufactured 5 coils on each conditions.
An Epstein test piece was collected from the cold-rolled annealed plate thus obtained, and the magnetic flux density B ₅₀ and the iron loss W _15/50 were measured, and the average value and standard deviation of each were determined.

The measurement results are shown in Table 1. In addition, the rolling required time shown in Table 1 is based on the condition A, and is shown as an increase or decrease relative to the condition A.
From Table 1, the steel sheet hot-rolled under the conditions suitable for the present invention has the same average magnetic property as that of the comparative example, but the variation is reduced to about 1/2. It can be seen that the worst value of the magnetic properties of the product plate is improved as compared with the comparative example. In addition, when rolling was performed by the method of the present invention, the rolling time was shortened by about 10 seconds compared with Comparative Example 1, and an advantageous effect was obtained in terms of productivity.

Claims (2)

C: 0.005 mass% or less, Si: 0.05 to 1.0 mass%, Mn: 1 mass% or less, Al: 1.0 mass% or less, P: 0.2 mass% or less, S: 0.005 mass% or less, and N : Containing 0.005 mass% or less, the remainder having a component composition consisting of Fe and inevitable impurities, and the component composition is obtained by reheating a slab having a γ-α transformation, and then performing rough rolling and finish rolling. In the method of manufacturing a non-oriented electrical steel sheet that is hot rolled, cold rolled, and finish annealed,
The total rolling reduction of the finish rolling is set to 93% or more, the rolling end temperature is set to the Ar ₁ transformation point or less,
Determine the rolling speed of the above finish rolling, calculate the lower limit value of the average cooling rate in the finishing mill necessary for rolling at the rolling speed, and then obtain the maximum sheet bar thickness at which the average cooling rate equal to or higher than the lower limit value is obtained. Then, the target sheet bar thickness is determined by calculating the minimum sheet bar thickness that satisfies the above-mentioned total rolling reduction in finish rolling, and based on those results, in rough rolling, the slab is rolled to the target sheet bar thickness and finished. In rolling, the sheet bar is rolled while being cooled at the calculated cooling rate. In addition to the above component composition, the slab further contains one or two selected from Sn: 0.005 to 0.1 mass% and Sb: 0.005 to 0.1 mass%. The manufacturing method of the non-oriented electrical steel sheet according to claim 1.