JP7151681B2 - Method for descaling Si-containing hot-rolled steel sheet and method for producing Si-containing cold-rolled steel sheet - Google Patents

Description

The present invention relates to a descaling method for hot-rolled steel sheets containing a large amount of Si, such as electrical steel sheets, and in particular for descaling hot-rolled steel sheets after hot-rolling annealing, which are raw materials for non-oriented electrical steel sheets and grain-oriented electrical steel sheets. and a method for producing a Si-containing cold-rolled steel sheet using the descaling method .

Electrical steel sheets, which are mainly used as iron core materials for electrical equipment, are roughly divided into non-oriented electrical steel sheets and grain-oriented electrical steel sheets. It is usually contained in large amounts. The above-mentioned non-oriented electrical steel sheet is produced by melting steel adjusted to a predetermined chemical composition, making it into a steel material (slab) by a continuous casting method or the like, hot-rolling the slab, and hot-rolled sheet if necessary. Annealing, pickling, and final annealing to recrystallize, and the grain-oriented electrical steel sheet is produced by melting steel adjusted to a predetermined chemical composition and made into a steel material (slab) by a continuous casting method or the like. After that, hot rolling is performed, and if necessary, the hot-rolled sheet is annealed, pickled, cold-rolled, subjected to primary recrystallization annealing that also serves as decarburization annealing, and after applying an annealing separator, secondary recrystallization. It is common to manufacture by giving finish annealing to make it.

Here, the hot-rolled sheet annealing is performed by subjecting the hot-rolled steel sheet (hot-rolled steel sheet) to homogenization heat treatment to eliminate insufficient recrystallization of the hot-rolled sheet and to reduce the crystal grains before cold rolling. This is done to improve the texture of the steel sheet before cold rolling to one that is advantageous to the magnetic properties and to prevent ridging by coarsening and regulating the grain size. High-grade electrical steel sheets are usually subjected to hot-rolled sheet annealing.

In pickling, when a steel sheet with oxide scale formed on the steel sheet surface by hot rolling or hot-rolled sheet annealing is cold rolled, the oxide scale is pushed into the steel sheet surface by the rolling rolls or peeled off. Oxidized scale adheres to the roll surface and causes surface defects when transferred, which significantly impairs the surface quality of the final product. is. As the pickling solution, any of hydrochloric acid, sulfuric acid, hydrofluoric acid, and a mixed acid of these acids is used.

The oxidized scale of the hot-rolled steel sheet for electrical steel sheet containing a large amount of Si and Al is an external scale such as FeO, Fe ₃ O ₄ , Fe ₂ O ₃ formed by the diffusion of Fe from the inside of the steel sheet to the outside. , Si oxides such as SiO ₂ and Fe ₂ SiO ₄ formed by diffusion of oxygen from the outside into the steel sheet, and subscales made of Al ₂ O ₃ and the like. In particular, subscales containing Al ₂ O ₃ are known to have remarkably poor descaling properties. Therefore, in the descaling of hot-rolled steel sheets for electrical steel sheets, mechanical descaling such as shot blasting is performed before pickling to improve the descaling performance.

However, in a high-grade electrical steel sheet with a high Si content, when hot-rolled sheet annealing is performed to ensure magnetic properties, the crystal grain size increases and the steel sheet becomes embrittled, which is a problem. In particular, when the Si content exceeds 2.5 mass%, the surface of the steel sheet undergoes twinning deformation due to shot blasting performed to improve descaling properties. There is a problem that troubles such as cracking are likely to occur.

As a method for solving the above problem, Patent Document 1 discloses that the diameter of shot particles projected by shot blasting is reduced to less than 0.35 mm, and the energy of the particles that collide with the steel plate surface is reduced. Techniques for reducing the amount of strain applied and suppressing twinning deformation have been disclosed. Further, in Patent Document 2, when shot blasting an electrical steel sheet containing 2.5 to 3.5 mass% Si, the steel sheet temperature T _s is changed according to the Si content by the following formula (1);
T _s (° C.)≧80×Si (mass%)−180 (1)
A technique for suppressing deformation twinning, which tends to occur under low-temperature, high-strain rate conditions, is disclosed by heating so as to satisfy the following.

JP-A-61-126919 JP-A-62-048463

However, in the technique of Patent Document 1, deformation twinning can be suppressed by making the shot particle diameter finer and reducing the collision energy. . In addition, in the technique of Patent Document 2, twinning deformation can be suppressed by raising the steel sheet temperature, but the higher the Si content, the higher the required steel sheet temperature, which is said to be the limit of rolling possibilities. A steel sheet containing 4.0 mass % Si has a heating temperature T _s of 140° C. obtained from the equation (1), which poses a problem of increased risk of dust explosion due to shot particles. Therefore, the actual situation is that it is difficult to apply the techniques of Patent Documents 1 and 2 to actual production.

Therefore, an object of the present invention is to provide a descaling method capable of preventing plate breakage due to twinning deformation during cold rolling without causing dust explosion in shot blasting and without deteriorating descaling properties. , to propose a method for producing a Si-containing cold-rolled steel sheet using the descaling method .

In order to solve the above problems of the prior art, the inventors focused on the material of the shot particles used for shot blasting and conducted extensive studies. As a result, by shot blasting using stainless steel shot particles, shot blasting can be performed without the risk of dust explosion even if the steel plate temperature is raised to 150 ° C. or higher, and descaling can be performed. The inventors have found that it is possible to prevent sheet breakage during cold rolling due to twinning deformation without deterioration, and have developed the present invention.

That is, the present invention provides a method for descaling a Si -containing hot-rolled steel sheet, wherein the steel sheet temperature is set to 150° C. or higher, the steel sheet surface is subjected to shot blasting using stainless steel shot particles, and then pickled. We propose a method for descaling a Si-containing hot-rolled steel sheet characterized by:

The average particle diameter of the shot particles used in the descaling method for Si-containing hot-rolled steel sheet of the present invention is characterized by being 0.3 mm or more and 0.7 mm or less.

Further, the Si-containing hot-rolled steel sheet in the descaling method of the Si-containing hot-rolled steel sheet of the present invention is characterized by containing Si in the range of 2.5 to 4.0 mass%.

Further, the Si-containing hot-rolled steel sheet in the descaling method of the Si-containing hot-rolled steel sheet of the present invention is characterized in that it is a steel sheet after hot-rolled sheet annealing.

Further, the descaling method of the Si-containing hot-rolled steel sheet of the present invention is characterized by using ceramic shot particles in place of the stainless steel shot particles.
Further, the present invention proposes a method for producing a Si-containing cold-rolled steel sheet, characterized by descaling the steel sheet by any one of the methods for descaling the Si-containing hot-rolled steel sheet described above, followed by cold rolling.

According to the present invention, even in a hot-rolled steel sheet containing a large amount of Si, such as an electrical steel sheet, by using stainless steel shot particles, even if the steel sheet temperature is high, the safety such as dust explosion is not impaired. Since shot blasting can be performed without descaling, deformation twinning introduced into the surface layer of the steel sheet by shot blasting is prevented, and rolling troubles such as plate breakage in the next cold rolling process are prevented. can be prevented.

It is a diagram showing the equipment configuration of a continuous annealing-pickling line to which the present invention can be applied. 4 is a photograph showing the effect of steel sheet temperature on structural changes in the steel sheet surface layer due to shot blasting. It is a schematic diagram of a blow-up type dust explosion test apparatus. 4 is a graph showing the relationship between the steel sheet temperature during shot blasting and the rate of occurrence of sheet breakage during cold rolling.

FIG. 1 shows the configuration of a continuous annealing-pickling line for hot rolled steel strips as an example of equipment to which the present invention can be applied. This line includes a payoff reel 2 for unwinding a coil of hot-rolled steel strip and paying out a steel strip 1 on the entry side, a welding machine 3 for welding preceding and following steel strips, and a heating strip 4 for heating the steel strip. , a soaking zone 5 for holding the steel strip at a predetermined temperature, a cooling zone 6 for cooling the steel strip, a shot blasting device 7 for performing mechanical descaling on the steel strip after annealing, a pickling bath 8 for descaling with an acid solution, and a tension reel 9 for winding the steel strip into a coil.

Usually, the hot-rolled steel strip 1 for electromagnetic steel sheet discharged from the payoff reel 2 is homogenized and annealed at a temperature of about 900 to 1100° C. in the soaking zone 5 from the latter half of the heating zone 4, and then the cooling zone 6. is cooled to a temperature of 100.degree. The steel strip 1 mechanically descaled by shot blasting is then completely descaled by pickling in a pickling tank 8, wound into a coil by a tension reel 9, and sent to the next cold rolling process. and sent.

Shot blasting is generally performed using a centrifugal rotor-type mechanical projection device, and the energy of shot particles projected onto the steel plate surface is controlled by adjusting the rotation speed of the rotor. In addition, by adjusting the mass of the shot particles projected per unit time according to changes in the line speed and the width of the steel strip, the projection density on the surface of the steel strip is controlled to be constant. The shot particles to be projected onto the steel plate surface are generally inexpensive Fe-based (carbon steel-based) shot particles having an average particle diameter of 0.5 to 1.0 mm.

Here, the present invention is characterized in that the temperature of the steel plate is set to 150° C. or higher and shot particles made of stainless steel are used when performing the shot blasting. The reason for this will be specifically described below.

In FIG. 2, a test piece was obtained from a hot-rolled steel sheet for electromagnetic steel containing 3.4 mass% of Si and subjected to hot-rolled sheet annealing at 920 ° C. for 30 sec. 1 is a photograph of a cross-sectional structure in the vicinity of a surface layer of a steel sheet when shot blasting is performed using steel particles of 0.45 mm as shot particles at a blasting speed of 60 m/sec and a blasting density of 12.5 kg/m ² . At this time, the steel plate temperature was changed to three levels of 20°C, 100°C and 150°C. Here, the average particle size is calculated from a histogram of volume-based frequencies measured with a laser diffraction particle size distribution measuring device, as specified in JIS Z 8819-2:2001. From this figure, it can be seen that deformation twinning occurs in the surface layer portion when the steel plate temperature is 20°C and 100°C, whereas deformation twinning is suppressed when the steel plate temperature is 150°C. I understand.

The temperature at which twinning deformation can be prevented for a steel sheet containing 3.4 mass% Si, which is obtained from the formula (1) of Patent Document 2, is 92°C. there is This cannot be determined because the shot blasting conditions (projection speed and projection density) are not specified in Patent Document 2, but it is considered that Patent Document 2 performed shot blasting with a smaller projection energy than the above experiment. . From this, it can be seen that when the shot particle projection energy is increased in order to improve the descaling property, sufficient effect for preventing deformation twinning may not be obtained simply by satisfying the formula (1) of Patent Document 2. all right.

Next, Table 1 shows the test pieces of hot-rolled steel sheets with the Si content changed to 6 levels in the range of 2.5 to 4.0 mass% after heating to a temperature of 150 ° C. The experiment shown in FIG. Shot blasting was performed under the same blasting conditions as in , and the cross section near the surface layer of the steel sheet was observed to investigate the occurrence of twinning deformation. From this table, it was found that by setting the steel sheet temperature to 150° C., the occurrence of deformation twinning could be suppressed in the entire Si content range of 2.5 to 4.0 mass %. Therefore, based on the above experimental results, the present invention targets hot-rolled steel sheets having a Si content of 2.5 to 4.0 mass%, and sets the lower limit of the steel sheet temperature to 150° C. when performing shot blasting. The upper limit of the steel plate temperature was set to 250° C. in order to prevent dust explosion during shot blasting, as will be described later.

Next, the reason why the material of the shot particles used in shot blasting, which is another feature of the present invention, is stainless steel will be described.
In normal shot blasting, the steel sheet temperature is often limited to 100° C. or less. This is due to the disaster prevention constraint of preventing dust explosions in shot blasting. That is, the technique of Patent Document 2 suppresses deformation twinning by increasing the temperature of the steel sheet, but has the risk of causing a dust explosion.

Therefore, the inventors conducted an experiment to investigate the influence of the material of shot particles on the explosiveness of dust.
FIG. 3 is a schematic diagram of a test apparatus for a blowing dust explosion test (JIS Z 8818) for evaluating the possibility of powder dust explosion. In this test device, test powder placed at the bottom of a transparent glass container is scattered inside the glass container by air supplied from a compressor, and an explosion is observed when it is ignited by a spark electrode installed inside the glass container. The powder is judged to be explosive when the flash of light from the glass container reaches a predetermined height or higher. Various variations in dust concentration are possible.

Table 2 shows that three types of shot particles with an average particle diameter of 0.3 mm and made of carbon steel, SUS304, and SUS430, which are made of carbon steel, SUS304, and SUS430, were tested using the above-described test equipment, and the dust concentration was changed stepwise from 500 to 6,000 g/m ³ . It shows the results of evaluating the dust explosiveness by changing the range. The evaluation of dust explosiveness was started from a condition where the dust concentration was low, and when dust explosions did not occur twice consecutively under the same conditions, it was determined as "no dust explosiveness". From this table, shot particles made of carbon steel cause dust explosions at dust concentrations of 1,500 g/m ³ or more, but typical stainless steels SUS304 and SUS 430 are explosive even at dust concentrations of 6,000 g/m ³ . Therefore, it was found that by using stainless steel shot particles, the risk of dust explosion can be greatly reduced even if the temperature of the steel plate is increased.

The reason why dust explosions can be suppressed by shot blasting using stainless steel shot particles is that even if the temperature of the shot particles rises, the Cr component contained in the stainless steel creates a dense and extremely thin chromium on the particle surface. It is believed that the oxide is formed and serves as a protective film to suppress subsequent oxidation, thereby suppressing heat generation due to oxidation.

Therefore, the stainless steel used as the raw material for the shot particles should be ferritic stainless steel typified by JIS SUS304 and austenitic stainless steel typified by SUS430 as long as it contains 10% by mass or more of Cr. In addition, austenitic/ferritic stainless steel, martensitic stainless steel, and the like may be used. A more preferable Cr content is 16 mass % or more.

Furthermore, the shot particles used in the present invention are preferably made of stainless steel and have an average particle diameter of 0.3 mm or more and 0.7 mm or less. If it is less than 0.3 mm, the collision energy is too small to obtain sufficient descaling properties. On the other hand, if it exceeds 0.7 mm, the collision energy becomes too large, so even if the steel plate temperature is 150 ° C. or higher, the introduction of twinning deformation to the steel plate surface layer cannot be suppressed, and cold rolling is difficult. This is because it becomes impossible to prevent plate breakage. A preferred average particle size is in the range of 0.3 to 0.5 mm.

In the above description of the present invention, the shot particles used for shot blasting are made of stainless steel. , Ni-based superalloys, and ceramics such as alumina (Al ₂ O ₃ ) and zirconia (ZrO ₂ ) may be used. However, shot particles made of general-purpose stainless steel are the most cost-effective.

In addition, even when shot blasting is performed using ceramic shot particles, the inventors heated the steel sheet temperature to a temperature of 150 ° C. or higher, and the average particle diameter was 0.3 to 0.7 mm. By using the ceramic shot particles, it is possible to prevent plate breakage during cold rolling without causing dust explosions and descaling performance, as with stainless steel shot particles. I am sure it can be done.

As described above, in the present invention, shot blasting at a high temperature is possible by using stainless steel shot particles, so that twinning deformation of the surface layer of the steel sheet can be suppressed, and sheet breakage in cold rolling can be prevented. etc. can be effectively prevented. The hot-rolled steel sheets targeted by the present invention include as-hot-rolled steel sheets and steel sheets obtained by subjecting the above steel sheets to hot-rolled sheet annealing. However, brittle fracture is likely to occur, and the oxide scale on the surface of the steel sheet is also strong. Therefore, the present invention is preferably applied to a steel sheet after hot-rolled sheet annealing in order to sufficiently exhibit the above effects.

In the continuous annealing-pickling line for hot-rolled steel strips for electrical steel sheets shown in FIG. A 2 mm Si-containing hot-rolled steel sheet was subjected to hot-rolled steel annealing under soaking conditions of 920°C for 30 seconds. At this time, the temperature of the hot-rolled steel strip after soaking was varied from 80° C. to 250° C. before shot blasting by controlling the flow rate of the cooling gas in the cooling zone. After that, in order to prevent dust explosion at high temperature, the surface of the steel plate was shot-blasted with shot particles made of SUS304 under the conditions of a particle velocity of 55 m/sec and a projection density of 12.5 kg/m ² . At this time, shot particles having an average particle size of 0.1 mm, 0.3 mm, 0.7 mm and 1.0 mm were used. In addition, it was confirmed that this shot blasting can be stably operated without causing a dust explosion even when the temperature of the steel plate is as high as 250°C.
Next, the steel plate after the shot blasting was pickled under the condition of being immersed in a hydrochloric acid aqueous solution (5 mass% HCl) at a temperature of 85°C for 10 seconds, and the presence or absence of scale remaining on the surface of the steel plate after pickling was confirmed using an optical microscope. .
Next, 30 coils of the pickled steel sheet for each condition were cold-rolled to a final sheet thickness of 0.3 mm using a tandem cold rolling mill, and the presence or absence of sheet breakage during cold rolling was investigated. did.

FIG. 4 is a graph showing the effects of the steel sheet temperature before shot blasting and the average particle size of shot particles on the rate of occurrence of sheet breakage in cold rolling. From this figure, it can be seen that at a steel plate temperature of 80°C (Comparative Example 1), which is the normal operating condition, the incidence of plate breakage is as high as 10% or more for any particle size. Although the rate of occurrence of plate breakage has decreased, at a steel plate temperature of 120°C (Comparative Example 2), twinning deformation can be prevented with a Si content of 3.7 mass%, which is obtained from the formula (1) described in Patent Document 2. It can be seen that although the steel sheet temperature is higher than 116° C., it is not possible to completely prevent sheet breakage during cold rolling. Further, when the steel plate temperature is 150°C or higher (invention example), plate breakage can be prevented when the shot particles have an average particle size of 0.1 mm, 0.3 mm, and 0.7 mm. It can be seen that even with the above, plate breakage cannot be completely suppressed. This is presumably because when the shot particle diameter was 1.0 mm, the twinning deformation of the surface layer of the steel sheet could not be suppressed due to the large projection energy.

Table 3 shows the results of investigating the presence or absence of scale residue on the surface of the steel plate after pickling. It can be seen that the descaling ability is lowered.

From the above experimental results, the steel plate temperature is heated to a temperature of 150 ° C. or higher, and shot blasting is performed using stainless steel shot particles having an average particle diameter of 0.3 to 0.7 mm. It was confirmed that sheet breakage during cold rolling can be prevented without causing descaling and without deteriorating descaling properties.

The technology of the present invention is preferably applied not only to descaling of Si-containing hot-rolled steel sheets, but also to steel sheets, such as stainless steel, high-carbon steel, and high-strength steel sheets, for which sheet breakage due to brittleness is a problem during cold rolling. be able to.

1: hot rolled steel strip 2: payoff reel 3: welding machine 4: heating zone 5: soaking zone 6: cooling zone 7: shot blasting device 8: pickling tank 9: tension reel

Claims (5)

In a method for descaling a Si-containing hot-rolled steel sheet,
The steel sheet temperature is set to 150 ° C. or higher, and the steel sheet surface is subjected to shot blasting using stainless steel shot particles having an average particle size of 0.3 mm or more and 0.5 mm or less , and then pickled. A descaling method for hot-rolled steel sheets. The method for descaling a Si-containing hot-rolled steel sheet according to claim 1 , wherein the Si-containing hot-rolled steel sheet contains Si in a range of 2.5 to 4.0 mass%. The method for descaling a Si-containing hot-rolled steel sheet according to any one of claims 1 and 2 , wherein the Si-containing hot-rolled steel sheet is a steel sheet after hot-rolled sheet annealing. The method for descaling a Si-containing hot-rolled steel sheet according to any one of claims 1 to 3 , wherein ceramic shot particles are used in place of the stainless steel shot particles. A method for producing a Si-containing cold-rolled steel sheet, comprising descaling by the method for descaling a Si-containing hot-rolled steel sheet according to any one of claims 1 to 4 , followed by cold rolling.

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