JP2021194662A - Method for producing special steel plate - Google Patents

Abstract

To provide a method for producing a special steel plate, capable of producing the special steel plate having a small dendrite inclination angle through casting by using a twin roll-type continuous casting device without performing a low speed casting.SOLUTION: A method for producing a special steel plate includes: casting a thin cast piece 5 by using a twin roll-type continuous casting device; hot-rolling the thin cast piece 5 to obtain a special steel plate material; defining a value of a coefficient a in advance for a relation between rolling reduction p during hot rolling when θ=0° and an inclination angle ω of dendrite in a steel plate, provided that θ is an inclination angle of dendrite in the thin cast piece 5, and then defining a critical rolling reduction pmax where recrystallization does not occur during the hot rolling; and adjusting the inclination angle θ of dendrite in the thin cast piece and the rolling reduction p within a range of p<pmax so that φC expressed by formula (1) satisfies -10°≤φC≤10°, where the formula (1) is as follows: φC=θ-a×p.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for manufacturing a special steel sheet.

From the viewpoint of process saving and energy saving, a technology for manufacturing a thin plate close to the final product at the casting stage, that is, near net shape continuous casting is being developed. Of these, the so-called strip continuous casting method is known as a promising thin plate-based near-net shape continuous casting. The strip continuous casting method is a near-net shape continuous casting method in which a molten metal and a mold roll (or belt) are brought into direct contact with each other to solidify and continuously cast to a casting thickness of about 0.1 to 10 mm (Non-Patent Document 1). Hereinafter, it is referred to as "continuous casting of thin-walled slabs". As a continuous casting method for thin-walled slabs, a double-roll type continuous casting method, a single-roll type continuous casting method, a double-belt type continuous casting method, a single-belt type continuous casting method, and the like are known.

In continuous casting of thin-walled slabs by a double-roll type continuous casting device, as shown in FIG. 1, a pair of casting rolls 1 are arranged and the closest contact distance between the rolls (gap of the minimum gap 20) is cast. It is the thickness of the slab 5. A fixed weir 2 (also referred to as a side weir) is pressed against both ends of the casting roll 1 to form a molten metal pool portion 3, and a pair of casting rolls 1 are continuously supplied with molten metal from a nozzle 4 to the molten metal pool portion 3. Rotate in opposite directions. A pair of solidified shells 23 formed along the peripheral surface of the roll are crimped at the closest portion (gap minimum portion 20) between the rolls to form a thin-walled slab 5. The peripheral speed of the casting roll 1 is the casting speed v of the thin-walled slab 5.

As described in Patent Document 1, in double-roll casting, it is known that the dendrite of the cast slab is tilted by 10 to 20 ° with respect to the perpendicular direction of the plate surface, and the rotation speed of the roll is increased. The more the dendrite tilts, the larger it is.

In the manufacture of special steel sheets, it is said that a steel sheet having excellent magnetic properties can be manufactured when the <100> orientations of the plate before cold rolling are aligned in the direction perpendicular to the plate surface. In Patent Document 1, dendrites in the cast structure of slabs are grown perpendicular to the surface of the solidified shell, and in the case of slabs having dendrites aligned in the direction perpendicular to the plate surface, the magnetic properties are changed. It is said that excellent steel sheets can be manufactured.
In Patent Document 1, a contact limiting plate for fixing the solidification start point of the molten metal is charged in the molten metal pool formed between the two rolls of the twin roll casting, and the arc length in which the molten pool contacts the rolls. Is disclosed, thereby reducing the peripheral speed (casting speed) of a casting roll when producing thin-walled slabs having the same thickness, and reducing the tilt angle of dendrites.

Japanese Unexamined Patent Publication No. 5-277657

5th Edition Steel Handbook Volume 1 Pig Iron and Steelmaking, pp. 456-457 Iron and Steel, Vol. 61, No. 14 (1975), pp. 298-2990

In the method described in Patent Document 1, the peripheral speed of the casting roll, that is, the casting speed is lowered in order to reduce the dendrite tilt angle of the slab, and the low-speed casting reduces the throughput of continuous casting and increases the productivity. It has the problem of getting worse.

Here, in the manufacture of special steel sheet, the material before cold rolling is referred to as "special steel sheet material". An object of the present invention is to provide a method for manufacturing a special steel sheet, which can manufacture a special steel sheet material having a small dendrite tilt angle without performing low-speed casting by casting using a twin-roll type continuous casting apparatus.

That is, the gist of the present invention is as follows.
[1] A method for manufacturing a special steel sheet, in which a thin-walled slab is cast using a double-roll type continuous casting apparatus and the thin-walled slab is hot-rolled to obtain a special steel sheet material.
The inclination of the dendrite of the slab is θ (the angle between the dendrite and the vertical line, and the casting direction is positive).
Regarding the relationship between the rolling rolling reduction rate p when θ = 0 ° and the slope ω of the dendrite of the steel sheet, the value of the coefficient a in the following equation (2) is determined in advance, and the limit for not recrystallizing by hot rolling. Set the rolling factor p _max ,
Using the coefficient a, the slope θ of the dendrite of the slab and the reduction rate p are set to the following (4) so that _{φ C} expressed by the following equation (1) satisfies -10 ° ≤ φ _{C ≤ 10 °.} A method for manufacturing a special steel sheet, which comprises adjusting within the range of the formula.
However, the rolling reduction ratio p (%) is obtained from the plate thickness x before rolling and the plate thickness y after rolling by the following equation (3).
φ _C = θ−a × p (1)
ω = −a × p (2)
p = (xy) / xx100 (3)
p <p _max (4)
[2] The method for manufacturing a special steel sheet according to [1], wherein the inclination θ of the dendrite of the slab is adjusted by adjusting the casting speed in the double-roll type continuous casting apparatus.

In the method for manufacturing a special steel sheet of the present invention, hot rolling is used by using double-roll continuous casting and hot rolling to adjust the inclination θ of the dendrite of the continuously cast slab and to adjust the rolling reduction ratio of hot rolling. It is possible to manufacture a special steel sheet of good quality by adjusting the inclination φ of the dendrite to the optimum range in the special steel sheet material after rolling.

(A) is a conceptual diagram showing the situation of double-roll type continuous casting, and (B) is an enlarged view of a cross section of a thin-walled slab. It is a conceptual diagram which shows the inclination θ of dendrite in a slab (solidification shell). It is a figure which shows the relationship between the rolling ratio p, and the slope ω of dendrite after rolling when a slab (inclination θ = 0 ° of dendrite) is rolled at the rolling ratio p. It is a figure which shows the relationship between the hot rolling reduction rate and the crystal grain size after hot rolling.

In a double-roll type continuous casting apparatus using a pair of rotating casting rolls, as shown in FIG. 1, a pair of casting rolls 1 is rotated to form a solidification shell 23 on the casting rolls 1 while thin-walled slabs 5 are formed. Is continuously cast. The pair of cast rolls 1 are close to each other in distance from the top to the bottom, and form the minimum gap 20 at the closest portion. Further, it has fixed weirs 2 in contact with both ends of the casting roll 1, and a molten metal pool portion 3 surrounded by the casting roll and the fixed weir 2 is formed above the gap minimum portion 20 of the pair of casting rolls 1. do. When the molten metal is supplied into the molten metal pool portion 3 via the nozzle 4, the molten metal solidifies at the portion in contact with the casting roll 1 to form the solidified shell 23. By rotating the casting rolls 1 in opposite directions, the solidification shell 23 formed on the surface of the casting roll moves together with the casting roll 1, and the solidification shells 23 on the surfaces of both casting rolls are crimped at the minimum gap 20. The thin-walled slab 5 is formed, and the thin-walled slab 5 is discharged downward from the minimum gap portion 20.

The circumferential speed of the surface of the casting roll 1 becomes the traveling speed of the solidified shell 23 in the molten metal pool portion 3, and at the same time, the casting speed v of the thin-walled slab 5. A hot water flow also exists in the molten steel of the molten metal pool portion 3, and FIG. 2 is a conceptual diagram showing a situation in which the solidified shell 23 runs in the casting direction 24. The solidified shell surface 25 is in contact with a casting roll (not shown). At the solid-liquid interface 26 between the solidified shell 23 and the molten steel pool, the difference between the molten steel flow velocity w in the molten metal pool portion and the traveling speed (casting speed v) of the solidified shell 23 is the relative speed V (solidified shell 23) between the two. It becomes the molten steel flow velocity seen from the entwined.

It is known that when a relative velocity V exists between the solidified shell 23 and the molten steel pool at the solid-liquid interface 26 of the solidified shell 23, a slope occurs in the growth direction of the dendrite 27 to be formed, as shown in FIG. There is. In the thin-walled slab (solidified shell 23 in FIG. 2), the angle between the direction of the dendrite 27 and the perpendicular to the surface of the steel plate is defined as the slope θ of the dendrite. The larger the relative velocity V, the larger the slope θ of the dendrite. It is known that the direction of the relative velocity V and the direction of the inclination of the dendrite are opposite to each other. The relationship between the relative velocity V and the dendrite slope θ has been clarified by Okano et al. Details will be described later.

Here, in the present invention, the angle of inclination of the dendrite of the thin-walled slab is defined as the angle formed by the vertical line on the surface of the thin-walled slab in the casting direction 24 as positive and the reverse direction as negative (see FIG. 2).

Normally, as the traveling speed (casting speed v) of the solidified shell 23 increases, the relative speed V also increases. Therefore, in the double-roll type continuous casting, the inclination θ of the dendrite of the slab can be changed by changing the casting speed v. Since the relative speed V is in the opposite direction to the casting speed v, as shown in FIG. 2, the inclination of the dendrite 27 faces the casting direction 24, and the inclination θ is a positive value.

In roll rolling of a plate material, if a vertical line perpendicular to the plate surface is marked in advance on the plate material before rolling, or if a slab whose dendrite is perpendicular to the plate surface is used and the reduction is performed at a predetermined reduction rate. As the rolling progresses, the surface of the material gradually advances beyond the center, and the original perpendicular line leaves the roll while being bent. As a result, when looking at the region from the surface to the 1/4 thickness below the surface in the cross section of the plate after rolling, the scribe line (dendrite) (perpendicular to the plate surface) provided before rolling is the vertical line of the plate. It will have a slope of ω in between. The slope ω changes depending on the reduction rate p, and the larger the reduction rate p, the larger the slope ω. The inclination direction of the inclination ω is opposite to the rolling direction. When the casting direction and the rolling direction are the same, the slope ω has a negative value.

As is clear from the above, in the production of thin steel sheets using double-roll continuous casting and hot rolling (the casting direction and the rolling direction are the same), the inclination was θ = 0 ° before rolling. After rolling one piece of dendrite, the inclination of the dendrite becomes ω, and the inclination ω (negative value) can be adjusted by adjusting the rolling ratio of hot rolling. Further, it is possible to variously adjust the inclination θ (positive value) of the dendrite before rolling by adjusting the casting speed and the like of the double roll continuous casting.

Therefore, in the present invention, in the method for manufacturing a special steel sheet, double-roll continuous casting and hot rolling are used, and the inclination θ of the dendrite of the continuously cast slab is adjusted and the rolling reduction ratio of the hot rolling is adjusted. When the inclination φ of the dendrite after hot rolling was adjusted to the optimum range to make a special steel sheet material, I thought that it would be possible to manufacture a special steel sheet of good quality.

<< Dendrite tilt control by hot rolling (using slabs with θ = 0 °) >>
A hot rolling test was conducted to investigate the effect of hot rolling on the inclination of dendrites. Using a mold immersion experiment, a single-sided sample of special steel with a dendrite inclination θ of the slab of approximately 0 ° was prepared. The sample was hot-rolled, and the effect of the rolling reduction rate p on the dendrite slope ω (≈φ) was investigated. The cast special steel was heated at 900 ° C. for 5 minutes and then hot-rolled. In order to investigate the rolling reduction p, the plate thickness of the slab was measured in advance with a micrometer, measured even after hot rolling, and the rolling reduction p was calculated using Eq. (3). Further, the L cross section (parallel to the casting direction and the plate thickness direction) of the obtained sample is cut, polished, and subjected to picric acid etching. The inclination ω of the dendrite was obtained by measuring and averaging 20 points of the inclination of the dendrite in the 1/4 thickness portion from the surface. The results are shown in FIG. As is clear from FIG. 3, the inclination ω of the dendrite increases in the negative direction as the reduction rate p increases.

When the relationship between the reduction rate p (%) and the slope ω (°) is expressed by the following equation (2) (a is a constant), the coefficient a changes depending on the steel type, so it should be obtained in advance as in this test example. Is preferable. In various tests by the inventor, for example, in the steel grade used in FIG. 3, the slope 0.87 obtained from these data by the least squares method was used as the coefficient a applied to the steel grade. For different steel types, the coefficient a can be obtained by the same method.
ω = −a × p (2)
p = (xy) / xx100 (3)

<< Dendrite tilt control by combining double-roll continuous casting and hot rolling >>
Special steel was cast using a twin roll casting apparatus having a pair of casting rolls having a casting roll width of 400 mm and a diameter of 600 mm as shown in FIG. The casting arc angle η was fixed at 40 °. The casting speed was 20 to 100 m / min. The faster the casting speed, the thinner the slab thickness, and the slab thickness d is substantially inversely proportional to the square root of the casting speed v.
The inclination θ of the dendrite of the thin-walled slab 5 at each casting speed was measured in advance. A slab cast by the double roll method is collected, and an L cross section (parallel to the casting direction and the plate thickness direction) at the center of the width is cut out. After that, the cross section is embedded, polished, and subjected to picric acid etching. Then, the inclination of the dendrite at the position of 1/4 thickness from the surface of the slab was measured at 20 points. As for the measurement location, the inclination of an arbitrary dendrite was selected and measured, and then the adjacent dendrites were repeatedly measured, and a total of 20 points were measured. The average value of 20 points was defined as the slope θ of the dendrite.
The crystal grain size after casting was about 100 μm regardless of the casting speed.

Then, hot rolling was performed, and rolling was performed so that the thickness of the thin steel plate (special steel plate material) after hot rolling was 1 mm. The casting direction and the rolling direction are the same.
The L cross section of the obtained thin steel plate (special steel plate material) sample was observed, and the inclination φ of the dendrite was measured. The method for measuring φ is the same as the method for measuring θ and ω described above.
Further, the crystal grain size was measured by the section method (JIS G0551) for the same L cross section, and the presence or absence of recrystallization was confirmed. The cross section of the sample is subjected to nital etching, and one straight line with a total length of 2 mm is drawn parallel to the plate surface at an arbitrary position 1/8 thick from the plate surface of the slab, and the number of crystal grains crossed by this straight line. Find n. The number of crystal grains whose ends of the straight line are inside is counted as (1/2). 2 mm was divided by the number of crystal grains n to determine the crystal grain size. Since a large amount of strain due to rolling enters the surface layer, the range from the surface layer to approximately 1/4 thickness is recrystallized. In the present invention, the average crystal grain size at the position 1/8 thick from the surface layer was determined by the above method. The change in crystal diameter due to recrystallization is considerably large, and it can be sufficiently determined by visual observation whether or not recrystallization has occurred. There is no particular critical value for the crystal diameter. For example, as shown in this example, what had an average diameter of 100 μm after casting becomes about 20 μm on average when recrystallized. The reduction rate when the average particle size changes rapidly in this way is defined as the limit reduction rate p _max . Since the critical rolling reduction rate varies depending on the steel type and rolling conditions, it is measured in advance as necessary. As shown in FIG. 4, the special steel recrystallized when a reduction of 30% or more was applied.

Table 1 shows the manufacturing conditions and manufacturing results. As is clear from Table 1, it can be seen that as the casting speed v increases, the slab thickness d becomes thinner and the dendrite inclination θ of the slab becomes larger.

Based on the results shown in Table 1, the relationship between the slopes θ and φ and the reduction rate p was first investigated.
By substituting a = 0.87 into the above equation (3), ω was calculated based on the equation (3) for each example. Next, the interrelationship between θ, ω and φ was evaluated. as a result,
φ = θ + ω (5)
It turned out that the relationship was established. Substituting equation (3) into this equation
φ = θ−a × p (6)
Is established. Equation (6) is an empirical formula showing the relationship between θ, p and φ.

Further, as shown in "Dendrite tilt angle ω calculated value" in Table 1, the value (ω = φ−θ) obtained by subtracting the dendrite tilt angle θ of the slab from the dendrite tilt angle φ after hot spreading is obtained from Eq. (2). Comparing the dendrite tilt angles (ω = -a × p), they corresponded almost one-to-one. From this, it was considered that the dendrite tilt angle after hot rolling can be predicted by Eq. (6).

The above equation (6) is an empirical equation, and represents an experimental result that the value on the right side of the equation (6) matches φ. _{When carrying out the present invention based on the experimental results, φ C} was introduced as a variable indicating the value on the right side of Eq. (6).
φ _C = θ−a × p (1)
(1) is defined. Then, θ and p are adjusted so that φ _C calculated from Eq. (1) is −10 ° ≦ φ _C ≦ 10 °.

_{That is, φ C} is adjusted based on the equation (1) by adjusting the inclination θ of the dendrite of the slab and adjusting the rolling reduction p in the hot rolling after the continuous casting by using the double roll type continuous casting. However, it has become possible to manufacture thin steel sheets (special steel sheet materials) with a small dendrite tilt angle. Here, the value of a in the formula (3) is determined in advance for the varieties to be produced by experiments.

_{Further, by obtaining the critical reduction rate p max} that does not recrystallize in advance and satisfying the following equation (4), an unrecrystallized special steel sheet material can be obtained. As a result, it is possible to manufacture a high-quality special steel sheet while achieving both a small dendrite tilt angle and productivity. In the two steel types used in this test, the critical reduction rate p _max was almost equivalent to 30%. The critical reduction rate p _max may be obtained for each steel type.
p <p _max (4)

In the thin-walled slab after continuous casting, the direction of the dendrite is the <100> direction. Therefore, when the solidified structure remains without recrystallization in hot rolling, the dendrite direction is considered to be the <100> direction even in the special steel sheet material after hot rolling. However, when recrystallized by hot rolling, new crystal grains are generated and grown from the inside, and the solidified structure is completely destroyed. Therefore, the inclination of the dendrite and the <100> direction do not match, and the crystal orientation Will be random.

In the examples shown in Table 1, the present invention example No. For 1 to 13, the φ _{C of} Eq. (1) is within the range of -10 ° to 10 °, the reduction rate p satisfies Eq. (4), and as a result, the undrite tilt angle φ after hot rolling is It was possible to obtain a special steel sheet material that was in the range of -10 ° to 10 ° and was not recrystallized.

On the other hand, Comparative Example No. In 1 to 8, _{both φ C of} equation (1) and the dendrite tilt angle φ after hot rolling were out of the range of −10 ° to 10 °. In addition, Comparative Example No. In 1 to 4, 6 and 7, the reduction rate did not satisfy the formula (4), and recrystallization was proceeding in the special steel sheet material. Numerical values outside the scope of the present invention are underlined.

<< Estimation of dendrite slope θ by molten steel flow >>
It is known by Okano et al. That the slope θ of the dendrite of a normal continuously cast slab follows equations (7) and (8) (see Non-Patent Document 2). Here, V is the molten steel flow velocity (relative velocity between the molten steel and the solidified shell), and f is the solidified rate. By transforming this equation, it can be expressed by equations (7)'and (8)'. Further, as shown in FIG. 2, the relative speed V between the solidified shell and the molten steel can be expressed as the equation (9) by the casting speed v and the molten steel flow velocity w of the downward flow. When the molten steel flow velocity (relative velocity V) is in the range of 2 to 100 cm / s, the equations (7)'and (8)' can be applied. Further, the solidification rate f is affected by the thickness of the cast slab and the like, but f = 0.01 to 1 cm / s.

Based on these findings, it is possible to estimate the dendrite slope θ using this equation even for thin-walled slabs, depending on the casting conditions.
lnV = (θ + 9.73 · lnf + 33.7) / (1.45 · lnf + 12.5) v <50 (7)
lnV = (θ + 4.83 ・ lnf + 7.2) / (0.1 ・ lnf + 5.4) v ≧ 50 (8)
θ = lnV × (1.45 ・ lnf + 12.5) − (9.73 ・ lnf + 33.7) v <50 (7)'
θ = lnV × (0.1 ・ lnf + 5.4) − (4.83 ・ lnf + 7.2) v ≧ 50 (8)'
V = v-w (9)

1 Casting roll 2 Fixed dam 3 Molten metal pool part 4 Nozzle 5 Thin-walled slab 20 Gap minimum part 23 Solidification shell 24 Casting direction 25 Solidification shell surface 26 Solid-liquid interface 27 Dendrite v Casting speed w Molten steel flow velocity V Relative speed θ Dendrite tilt φ Dendrite tilt of steel plate ω Dendrite tilt of steel plate (when the dendrite tilt of the slab is 0 °)
η Casting arc angle

Claims (2)

It is a method for manufacturing a special steel sheet in which a thin-walled slab is cast using a double-roll type continuous casting apparatus and the thin-walled slab is hot-rolled to be a special steel sheet material.
The inclination of the dendrite of the slab is θ (the angle between the dendrite and the vertical line, and the casting direction is positive).
Regarding the relationship between the rolling rolling reduction rate p when θ = 0 ° and the slope ω of the dendrite of the steel sheet, the value of the coefficient a in the following equation (2) is determined in advance, and the limit for not recrystallizing by hot rolling. Set the rolling factor p _max ,
Using the coefficient a, the slope θ of the dendrite of the slab and the reduction rate p are set to the following (4) so that _{φ C} expressed by the following equation (1) satisfies -10 ° ≤ φ _{C ≤ 10 °.} A method for manufacturing a special steel sheet, which comprises adjusting within the range of the formula.
However, the rolling reduction ratio p (%) is obtained from the plate thickness x before rolling and the plate thickness y after rolling by the following equation (3).
φ _C = θ−a × p (1)
ω = −a × p (2)
p = (xy) / xx100 (3)
p <p _max (4) The method for manufacturing a special steel sheet according to claim 1, wherein the inclination θ of the dendrite of the slab is adjusted by adjusting the casting speed in the double-roll type continuous casting apparatus.

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