JP7687280B2 - Belly-attached caliber roll, method for manufacturing rolled material, method for manufacturing steel pipe, and rolling mill - Google Patents

Description

The present invention relates to a technology for rolling difficult-to-process steel materials such as high Cr steel and other high alloy steels that require high corrosion resistance and can be used as seamless steel pipe materials, steel bar materials, wire rod materials, etc., and in particular to a technology for suppressing the occurrence of defects on the surface of the resulting rolled material.

In a conventional blooming process, the material to be rolled, such as a slab, is first heated in a heating furnace to 800°C to 1300°C. In the present invention, the material to be rolled includes not only steel ingots, but also billets, blooms, slabs, and other cast pieces obtained by forging.

Next, the material to be rolled passes between the caliber rolls of a blooming mill, which are set above and below each other and have a specified gap between them, and is rolled, after which the material to be rolled is rotated 90 degrees in the circumferential direction.
The material to be rolled is moved in the opposite direction to the rolling direction, and reverse rolling, in which the rolls are also rotated in the opposite direction, is repeated until a bloom or other rolled material having a predetermined cross-sectional shape is obtained.
In reverse rolling, it is possible to obtain a desired cross-sectional dimension with about 5 to 8 passes for small cross-sectional dimensions, but in the case of rolling until a small cross-sectional dimension is obtained, more than the above rolling is required, and in some cases, reverse rolling of 20 passes or more is required.

As rolling progresses, the temperature of the rolled material decreases. For steels with excellent hot workability, such as carbon steel, it is possible to roll them to the desired cross-sectional dimensions with a single heating. However, for steels with poor hot workability, such as high Cr steel and other high alloy steels (hereinafter referred to as difficult-to-work steels), defects may form on the surface of the resulting bloomed rolled material, deteriorating the surface properties. In this case, defects formed on the surface, especially near the corners, can increase maintenance time and reduce yields.

Therefore, in order to suppress defects, component compositions have been proposed for improving hot workability in blooming of difficult-to-work materials (see, for example, Patent Documents 1 and 2).
Also, a method has been proposed in which the hot strength is reduced and hot workability is improved by increasing the working temperature range (see, for example, Patent Document 3).
Also, a hot working method including a specific forging process instead of blooming from a cast slab has been proposed (see, for example, Patent Document 4). Also, a rolling method using rolls with two or more different tapers on the slab contact surface has been proposed (see, for example, Patent Document 5).

Japanese Unexamined Patent Publication No. 61-201727 JP 2005-205454 A Japanese Patent Application Publication No. 1-262048 JP 2002-194431 A Japanese Patent Application Publication No. 5-76913

The addition of elements such as Mn, Ni, and B to improve hot workability, as in the techniques described in Patent Documents 1 and 2, not only increases costs, but also leaves the surface quality of the workpiece after rolling insufficient.

In addition, with regard to processing in high temperature regions as described in Patent Document 3, duplex stainless steels and ferritic stainless steels, which have a high ferrite fraction in high temperature regions, have a significant decrease in hot strength when exposed to high temperatures, which can cause sagging in the furnace or defects due to the furnace material biting into the steel material.
In addition, ferritic stainless steels have high hot workability and are therefore unlikely to develop rolling defects, whereas duplex stainless steels have ferrite and austenite phases with different strengths in the hot working temperature range, and therefore generally have low hot workability and are prone to developing rolling defects. Therefore, in materials with poor hot workability such as duplex stainless steels, the heating temperature cannot be increased, and the technology described in Patent Document 3 cannot be applied.

Furthermore, as in the technique described in Patent Document 4, the inclusion of a forging step as a pre-machining step increases the number of steps, which results in an increase in costs.
Furthermore, as in the technology described in Patent Document 5, if the roll shape causes the shape of the rolled material after rolling to be asymmetric, in blooming in which the material is reverse rolled while rotating in the circumferential direction, the material will not be horizontal to the transport path, which is a problem in that handleability will be poor.

As such, there was a need for improved technology that could prevent defects from forming in the resulting rolled material without adjusting the processing temperature or including a forging process.

The present invention was made to solve the problems of the conventional technology, and aims to provide a technology that suppresses the formation of defects in rolled material.

As a result of extensive research to solve the above problems, the inventors of the present invention focused on the shape of the caliber bottom (the bottom of the groove) of a caliber roll used for rolling, which comes into contact with the material being rolled, and discovered a caliber roll shape that prevents defects from being formed in the resulting rolled material.

A defect is formed when tensile stress acts on the vicinity of the site where the defect occurs. A rolled material such as a steel plate compressed in the thickness direction by a rolling roll elongates not only in the length direction but also in the width direction.
When the rolled material is stretched, the amount of stretching that occurs at the points of the rolled material in contact with the rolling rolls and at the center of the rolled material not in contact with the rolling rolls differs depending on the degree of friction with the rolling rolls, and the center of the rolled material stretches more.
As a result, the material being rolled deforms into a mountain shape on the contact surface with the rolling roll, and this mountain-shaped deformation is called bulging (hereinafter, bulge deformation). Bulge deformation varies depending on the roll diameter, reduction ratio, thickness and width of the material being rolled, frictional resistance, etc.

When the thickness of the rolled material is thin, the top of the bulge (the part where the deformation is greatest) is formed in the center of the thickness.
On the other hand, when the thickness of the rolled material is large and the reduction ratio is small, bulging occurs not in the center of the thickness but at both ends in the width direction of the rolled material. This type of bulging shape is called a double bulge.

It is believed that when this double bulge is formed, the portion where bulging occurs during rolling (the bulging portion) comes into contact with the rolling roll first and is rolled, so that tensile stress acts on the material, forming defects.
In particular, when a bulging portion occurs near a corner of the rolled material, the portion that is deformed by rolling of the bulging portion becomes a free deformation portion, and since there is little constraint on the surrounding area, multiaxial tension is likely to occur, resulting in the formation of defects at the corner.

The inventors focused on these problems in rolling and discovered that by adjusting the caliber bottom shape of the caliber roll (the shape of the bottom of the groove), it is possible to control the material flow (metal flow) in the bulging part of the rolled material, and to obtain a rolled material with reduced defects without causing multiaxial tension in the corners of the rolled material where defects are likely to form.

The present invention was completed based on these findings and through further investigation, and the gist of the present invention is as follows.
[1] A caliber roll for rolling a material to be rolled,
The roll has one or more grooves extending in a concave shape in the circumferential direction on the outer surface of the roll, into which the rolled material is fitted during the rolling,
The groove portion has a pair of side wall portions and a bottom portion adjacent to each of the pair of side wall portions as opposed to the surface of the rolled material to be fitted therein,
the bottom portion of at least one of the groove portions has a mountain-shaped structure inclined toward the rolled material side as it is spaced apart from each of the pair of side wall portions,
A caliber roll with berries, wherein the inclination angle θ of the mountain-shaped structure with respect to the roll axial direction satisfies the following formula (1).
0 degrees < θ ≦ 30 degrees ... (1)
[2] A method for producing a rolled material by rolling using the belly-equipped caliber roll according to [1],
In the rolling, the material to be rolled is heated to a heating temperature of 800°C or higher, and then passed between a pair of caliber rolls with bellies, with a rolling reduction rate per pass of 70% or less.
[3] The method for producing a rolled material according to [2], wherein Cr steel containing 5.0 mass% or more of Cr is used as the rolled material.
[4] A method for producing a steel pipe, comprising producing a steel pipe using a rolled material obtained by the method for producing a rolled material according to [2] or [3] above.
[5] A rolling mill for rolling a material to be rolled,
A pair of caliber rolls with bellies are disposed opposite to each other,
Each of the pair of caliber rolls with berries comprises:
The roll has one or more grooves extending in a concave shape in the circumferential direction on the outer surface of the roll, into which the rolled material is fitted during the rolling,
The groove portion has a pair of side wall portions and a bottom portion adjacent to each of the pair of side wall portions as opposed to the surface of the rolled material to be fitted therein,
the bottom portion of at least one of the groove portions has a mountain-shaped structure inclined toward the rolled material side as it is spaced apart from each of the pair of side wall portions,
A rolling mill, wherein an inclination angle θ of the mountain-shaped structure with respect to a roll axial direction satisfies the following formula (1).
0 degrees < θ ≦ 30 degrees ... (1)

The present invention can prevent defects from forming in the rolled material.

FIG. 1 is a diagram for explaining the cross-sectional shape of a material to be rolled (a slab). FIG. 2 is a diagram for explaining an outline of a rolling mill (blooming mill). FIG. 3 is a diagram for explaining the caliber roll with berries of the present invention. FIG. 4 is a diagram for explaining the inclination angle θ defined by the caliber roll with berries of the present invention. FIG. 5 is a diagram for explaining the mechanism of defect formation in the bulging portion when rolling is performed using the roll shapes of a conventional caliber roll and a caliber roll with a belly of the present invention.

The following describes an embodiment of the present invention with reference to the drawings.

First, before explaining the belly caliber roll of the present invention, we will explain the material to be rolled by the belly caliber roll of the present invention and the rolling mill (slabbing mill) that has the belly caliber roll of the present invention.

Figure 1 is a diagram for explaining the cross-sectional shape of the rolled material S, such as a slab, used in the present invention. As shown in Figure 1, the rolled material S has a rectangular shape when viewed in cross section perpendicular to the rolling direction, and has a first side and a second side adjacent to the first side, the length of which is equal to or greater than the length of the first side before the start of rolling in the first pass. The rolled material S is formed into a rolled material (blooming material) by rolling it using a rolling mill (blooming mill) having a caliber roll with a belly, which will be described later with reference to Figure 2.

FIG. 2 is a diagram for explaining a rolling mill (splashing mill) 1 having a caliber roll 10 with berries (hereinafter also simply referred to as the caliber roll 10) of the present invention.
The rolling mill (splashing mill) 1 has a pair of caliber rolls 10 with bellies arranged opposite to each other.
The rolling mill (slabbing mill) 1 is provided on the outer surface of a caliber roll 10, and performs rolling (slabbing) on the rolled material S by fitting the rolled material S into a groove portion (hole-shaped portion) formed by a flange capable of supporting the rolled material S.
The material S to be rolled can be heated to a heating temperature of 800°C or more in a heating furnace before being rolled by the rolling mill (blooming mill) 1, and then passes between a pair of caliber rolls 10 of the rolling mill (blooming mill) 1 shown in FIG. 2, whereby it is rolled in the rolling direction (see arrows R.D. in the figure). The material S to be rolled is then rotated 90 degrees in the circumferential direction, and rolled again while moving in the opposite direction to the above rolling direction. In this way, each time the material S to be rolled passes through the caliber rolls (calibre rolling rolls) 10, reverse rolling is repeated while the rolls 10 are rotated in the opposite direction, until the material S to be rolled becomes a rolled material (bloom) having a predetermined cross-sectional shape. It is preferable that the reduction rate per pass in all passes is 70% or less.
Here, the first side and the second side described with reference to Fig. 1 refer to the same parts after each pass even when rotated 90 degrees in the circumferential direction. It is sufficient that the length of the second side before the start of rolling in the first pass is equal to or longer than the length of the first side, and the length of the second side after the first pass may be equal to or shorter than the length of the first side.

Next, the details of the caliber roll 10 with berries of the present invention will be described. Fig. 3 is a cross-sectional view for explaining the caliber roll 10 with berries of the present invention (a cross-sectional view perpendicular to the bottom surface (circle) when the roll 10 is cylindrical, a cross-sectional view including the roll rotation axis (a cross-sectional view taken along the arrow A-A in Fig. 2)).
As shown in Figure 3, the belly-equipped caliber roll 10 of the present invention is a roll for rolling (slabbing) a workpiece S, and has one or more grooves 11 (11A, 11B, 11C, 11D) that are concavely extended in the circumferential direction (see arrow a) on the outer surface of the roll 10 and into which the workpiece S is fitted during rolling (slabbing), and each groove 11 has a pair of side wall portions 12 and a bottom portion 13 adjacent to each of the pair of side wall portions 12 as opposing surfaces for the workpiece S to be fitted into, and the bottom portion 13 of at least one of the groove portions 11 has a mountain-shaped structure that is inclined toward the workpiece S to be fitted into as it moves away from each of the pair of side wall portions 12, and the inclination angle θ of the mountain-shaped structure with respect to the roll axial direction satisfies the following formula (1).
0 degrees < θ ≦ 30 degrees ... (1)

The shape of the roll in the caliber roll 10 with berries of the present invention is not particularly limited as long as it can perform rolling (slabbing), and can be, for example, cylindrical. The material of the roll is also not particularly limited, and can be, for example, cemented carbide (an alloy of tungsten carbide and cobalt) or high-speed steel.

The "belly" of a caliber roll with a belly refers to the part at the bottom of the groove that protrudes toward the rolled material that is fitted into it, and the "belly" of the caliber roll 10 with a belly of the present invention refers to a specific mountain-shaped structure at the bottom 13.

The caliber roll 10 with berries of the present invention has one or more grooves (calibers, holes, recesses) 11, or may have only one groove. It may also have two or more grooves 11 in the roll width direction (roll axial direction, perpendicular to the rolling direction, X direction), and these grooves may have different shapes and sizes, but it is preferable that a pair of grooves 11 facing each other during rolling have the same shape and size.
In the example shown in Fig. 3, the caliber roll 10 has four grooves 11A, 11B, 11C, and 11D, and in the above-mentioned reverse rolling, the first pass rolling is performed in the groove 11A, the third and fifth passes rolling in the groove 11B, the seventh and ninth passes rolling in the groove 11C, and the second, fourth, sixth, and eighth passes rolling in the groove 11D, but the present invention is not limited to such an example, and it is sufficient to adopt a caliber roll suitable for the rolling conditions set for the shape of the rolled material (bloom rolled material) to be obtained. Specifically, the grooves into which the rolled material S is fitted can be appropriately selected or designed according to the length ratio between the first side and the second side of the rolled material S to be set in each pass.

The caliber roll 10 with bellies of the present invention is characterized by the shape of the grooves 11. The grooves 11 have a pair of side walls 12 and bottoms 13 adjacent to the pair of side walls 12 as opposing surfaces of the rolled material S to be fitted. In the caliber roll 10 with bellies, the bottoms 13 of at least one groove 11 have a mountain-shaped structure that is inclined toward the rolled material S side as it moves away from the pair of side walls 12, and the inclination angle θ of the mountain-shaped structure with respect to the roll axial direction (X direction) satisfies the formula (1) described later.
As shown in FIG. 3 , in the groove portion 11 of the roll 10 that satisfies the formula (1), inclined portions having the above-mentioned inclination angle θ are formed at two locations on both ends of the bottom portion 13 adjacent to each of a pair of side wall portions 12.
The inclination angles θ formed at both ends of the bottom may be different or the same, but from the viewpoint of handling during rolling, it is preferable that the difference between the inclination angles θ at both ends is within a range of 5 degrees or less. It is more preferable that the inclination angles θ formed at both ends of the bottom are the same. That is, in the cross-sectional view of Figure 3, in the above-mentioned mountain-shaped structure, it is preferable that the distance from the axial (X-axis) center position to the boundary positions on both sides of the bottom 13 and the side wall portion 12 is the same, and that the above-mentioned axial center position protrudes most toward the rolled material side (Z-axis side).
In addition, in a pair of caliber rolls 10, the inclination angles θ of the inclined portions located at the four corners formed in a pair of grooves 11 facing each other during rolling may be different from each other or may be the same, but from the viewpoint of handling during rolling, it is preferable that the difference in the inclination angles θ of the four corners is within a range of 5 degrees or less. It is more preferable that the inclination angles θ of the four corners are the same.
In the caliber roll 10 with berries of the present invention, at least one of the one or more groove portions 11 needs to have a mountain-shaped structure that satisfies formula (1), and the roll may have a groove portion that does not have a mountain-shaped structure (having a flat bottom (θ = 0 degrees)).
In particular, since defects are likely to be formed during an odd number of passes in which the reduction is large, it is preferable to perform rolling in an odd number of passes using grooves having a mountain-shaped structure.

0 degrees < θ ≦ 30 degrees ... (1)
Next, the reason for defining the formula (1) will be described.
Fig. 4 is a diagram for explaining the inclination angle θ defined in the caliber roll 10 with bellies of the present invention. Specifically, it is a partial cross-sectional view of the caliber roll 10 with bellies (a partial cross-sectional view perpendicular to the bottom surface (circle) when the roll 10 is cylindrical (a cross-sectional view taken along the line A-A in Fig. 2)).
As shown in FIG. 4, in the present invention, the inclination angle θ (θ ₁ , θ ₂ , θ 3 , θ ₄ ) of the mountain-shaped structure at the bottom of the caliber roll 10 with berries relative to the roll axial direction (X- _axis direction) satisfies the above formula (1).
In the following description, the inclination angles θ (θ ₁ , θ ₂ , θ ₃ , θ ₄ ) of the inclined portions located at the four corners formed in a pair of grooves 11 facing each other during rolling are all assumed to be the same angle, but may be different as long as they are within the range satisfying formula (1). In Fig. 4, the symbol H indicates the belly height, the symbol h indicates the caliber depth, and X1 indicates the flange height.

Here, first, an explanation will be given of the evaluation method and results of investigating whether the belly caliber roll 10 of the present invention suppresses defects at the corners of the rolled material (bloom rolled material).
The following Table 1 shows the evaluated rolling conditions. As the caliber roll with belly (belly roll) 10 of the present invention, a roll having a total of three grooves was used, including grooves for 1, 3, 5, and 7 passes, grooves for 9, 11, and 13 passes, and a groove for an even number of passes that does not have a mountain-shaped structure. In the grooves for 1, 3, 5, and 7 passes, θ=7 degrees, and in the grooves for 9, 11, and 13 passes that have different shapes and sizes from the grooves for 1, 3, 5, and 7 passes, θ=7 degrees. In the grooves for even passes, θ=0 degrees. Note that all four θ in each groove were the same.
As a comparative example outside the scope of the present invention, a conventional caliber roll was used in which all grooves had no berries, i.e., the bottoms (caliber bottoms) of all grooves were flat. In other words, the conventional caliber roll had a θ of 0 degrees in all grooves.
The material to be rolled was SUS329J1. Both the inventive example and the comparative example were rolled under the conditions shown in Table 1.

The length ratio of the rolled material shown in Table 1 is the length (mm) of the second side of the rectangle to the length (mm) of the first side of the rectangle, the length of which is adjacent to the first side and is equal to or longer than the length of the first side before the start of the first pass of rolling, after each pass of rolling (see Figure 1 again).

The rolled material heated to 1100°C in a heating furnace was rolled under the rolling conditions shown in Table 1. After rolling, the bloomed material was subjected to a non-destructive inspection of the depth of surface defects by a magnetic leakage flux testing method (hereinafter also referred to as MLFT) using a magnetic leakage flux detector manufactured by Forster, and the depth of defects of 3.0 mm or more was measured in the rolling direction, and the number of defects was counted and used as an evaluation item. More specifically, while rotating the bloomed material obtained, the peak intensity over the entire length in the rolling direction was obtained by MLFT, and the number of defects with a peak level of 3.0 mm or more was counted, and the number was divided by the entire length in the rolling direction to calculate the number of defects (defects/m).
The evaluation results by MLFT were 114.2 defects/m for the conventional caliber roll and 44.8 defects/m for the caliber roll with berries of the present invention. This confirmed that the formation of defects was suppressed by rolling using the caliber roll with berries under the conditions shown in Table 1. The more defects found by MLFT, the more time is required for treatment with a grinder or the like to remove the defects on the surface of the bloomed material, and the worse the yield becomes.

As described above, the occurrence of defects could be reduced by rolling using a caliber roll 10 with a belly.

Next, the reason why compressive stress, rather than tensile stress, acts on the portion (bulging portion) where defects are formed in the rolled material during rolling in the caliber roll with belly will be explained.
FIG. 5 shows a schematic diagram of the defect formation mechanism in the bulging portion of a conventional caliber roll and belly roll.
As shown in Fig. 5(a), in the conventional caliber roll 100, at the beginning of rolling, the bottom 130 of the groove 110 contacts the bulging part, causing metal flow to occur to the left and right from the contact point, and tensile stress acts on the material S to form defects. On the other hand, as shown in Fig. 5(b), in the caliber roll 10 with a belly, the bottom 13 of the groove 11 has a mountain-shaped structure that is inclined toward the side of the rolled material S fitted therein as it moves away from the side wall 12 (not shown in Fig. 5). That is, in the caliber roll 10 with a belly, the roll is inclined from the center of the roll (center in the axial direction (X-axis direction)) toward the side wall (flange) 12. Therefore, the metal flow of the bulging part is controlled to one side, thereby suppressing the tensile stress. That is, in the caliber roll 10 with a belly, the tensile stress is suppressed on the material to be rolled during rolling, and mainly compressive stress acts on the material to be rolled.
As described above, the belly caliber roll 10 of the present invention can reduce defects by controlling the metal flow in the bulging portion.

As a result of extensive investigation based on the above findings, the present invention specifies "0 degrees < θ ≦ 30 degrees" as formula (1). The reason for specifying this range will be explained below.
First, in formula (1), when θ = 0 degrees, this is synonymous with the caliber roll of the prior art described with reference to Figure 5 (a), and as described above, the suppression of the occurrence of scratches was not sufficient, so this is excluded.
Also, when θ<0 degrees, that is, as the bottom part moves away from each of the pair of side walls, it has a mountain-shaped structure inclined in a direction away from the fitted rolled material. With such a concave inclination toward the roll center, it is possible to obtain a similar effect in that the metal flow of the bulging part is brought to one side, but the shape of the rolled material becomes a mountain-shaped shape protruding toward the center of the side. Therefore, because of such a convex mountain-shaped shape, it is not possible to handle the rolled material with high accuracy during transportation, etc., and the rolled material may unintentionally roll over, causing damage to the equipment. For the above reasons, in the present invention, θ>0 degrees. Preferably, θ>1 degree, and more preferably, θ≧2 degrees.
Furthermore, if the inclination angle is smaller than that of the bulging portion formed by rolling, the force controlling the metal flow to one side will be weakened, so it is preferable that θ≦the inclination angle from the pre-estimated apex of the bulging portion to the flat portion where no bulging is formed.

Next, we will explain θ≦30 degrees. As the inclination angle θ increases, the inclination of the mountain-shaped structure of the bottom 13 becomes steeper, resulting in an acute-angled belly. When rolling with an acute-angled belly, the center of the rolled material is greatly depressed, and in the process of forming the depression, tensile stress acts toward the center of the material, leading to the formation of defects. And when θ exceeds 30 degrees, the desired effect of suppressing defects cannot be obtained. Therefore, in the present invention, θ≦30 degrees. Preferably, θ≦15 degrees, and more preferably, θ<10 degrees.

The measurement positions for θ are at both ends of the bottom portion 13 in the roll axis direction (X direction), and at the boundary positions with the adjacent side wall portions 12.

The rolled material S used in the manufacturing method of the rolled material (blooming rolled material) of the present invention can be difficult-to-process steel materials (difficult-to-process high alloy steels) such as Cr steel and stainless steel containing 5.0% or more by mass of Cr. Here, difficult-to-process steel materials refer to steel materials whose metal structure is in a two-phase or more state in the rolling temperature range, and specifically, stainless steels with high Cr contents such as 17% by mass Cr, 22% by mass Cr, and 25% by mass Cr can be mentioned. Although difficult-to-process steel materials are difficult to process compared to single-phase steel materials due to the difference in strength between the phases, the manufacturing method of the rolled material (blooming rolled material) of the present invention can suppress the occurrence of defects even when the rolled material S is such difficult-to-process steel materials.

The rolled material S is produced by the following process. First, pig iron is produced by melting iron ore and coke in a blast furnace at the same time. Then, the molten iron is pre-treated with desiliconization and desulfurization processes, and carbon is removed in a converter to produce molten steel. After secondary refining, which finely adjusts the components of the molten steel, such as the alloy elements required for the molten steel, the rolled material S is obtained by continuous casting. In addition to continuous casting, the rolled material S may also be produced by ingot casting.
In the present invention, not only steel ingots but also billets, blooms, slabs and other cast pieces obtained by forging can be used as the material to be rolled.

The material S to be rolled is preferably heated to a temperature of 800°C or higher and then passed between a pair of caliber rolls with berries (grooved rolling rolls) 10. In addition, it is preferable that the reduction rate per pass be 70% or less in all passes.

If the heating temperature is less than 800°C, the deformation resistance of the material S to be rolled increases and may exceed the load capacity of the roll. Therefore, the heating temperature is preferably 800°C or higher. More preferably, it is 1000°C or higher, and even more preferably, it is 1200°C or higher. On the other hand, the upper limit of the heating temperature is not particularly limited, but if it exceeds 1300°C, the deformation resistance of the material to be rolled is low because high Cr steel such as 22 mass% Cr has a high ferrite fraction, and the material may sag in the furnace. Therefore, the heating temperature is preferably 1300°C or lower. More preferably, it is 1290°C or lower, and even more preferably, it is 1250°C or lower.

Furthermore, if the reduction rate per pass exceeds 70%, uniform deformation behavior will not occur, and the material may crack diagonally, resulting in a phenomenon such as shear cracking. Therefore, the reduction rate is preferably 70% or less. Also, it is more preferably 50% or less, and even more preferably 35% or less. Also, if the reduction rate is too low, it will take too much time to achieve the specified dimensions and the temperature of the rolled material will drop, so it is more preferably 5% or more, and even more preferably 10% or more.

The above rolling reduction (%) is for each pass.
It is the larger value among ((length of first side before rolling (mm)−length of first side after rolling (mm))/length of first side before rolling (mm))×100 and ((length of second side before rolling (mm)−length of second side after rolling (mm))/length of second side before rolling (mm))×100.
The above-mentioned first side length and second side length are the lengths of the sides of a rectangle when viewed in a cross section perpendicular to the rolling direction of the rolled material (see FIG. 1 again).

In addition, in the present invention, steel pipes can be manufactured using the rolled material obtained by the above-mentioned method for manufacturing rolled material.

The manufacturing conditions for steel pipes are preferably as follows: round steel billets are used as the rolled material (splashing material), which is heated to 1100-1300°C in a heating furnace and turned into a hollow blank tube using a Mannesmann piercing machine. The hollow blank tube is rolled in a mandrel mill to reduce the outer diameter and thickness to make a long blank tube. This is then held at 700-1000°C for one hour in a reheating furnace, reheated, and then cut to the finished dimensions using a stretch reducer, before being cooled, straightened, and cut to make the steel pipe.

To investigate the defect suppression technology of this invention, the present invention conditions and comparative conditions were prepared, and the MLFT results after rolling were investigated.

The material to be rolled was SUS329J1. The initial cross-sectional dimensions of the material to be rolled were first side: 275 mm x second side: 710 mm. The material to be rolled was heated to 1100°C in a heating furnace and then rolled. Other conditions are shown in Table 1. Table 2 shows the results of each inclination angle θ of the Caliber roll and the number of defects of 3.0 mm or more by MLFT.
More specifically, in measuring the defects, while rotating the bloomed and rolled material obtained, the peak strength over the entire length in the rolling direction was obtained by MLFT, and the number of defects with a peak level of 3.0 mm or more was counted. The number was divided by the entire length in the rolling direction to calculate the number of defects (defects/m).
For each level, a roll having three grooves in total was used: grooves for 1st, 3rd, 5th and 7th passes, grooves for 9th, 11th and 13th passes, and a groove for an even number of passes without a chevron structure.
The angles θ of the grooves for passes 1, 3, 5, and 7, and the angles θ of the grooves for passes 9, 11, and 13 were all the same as shown in Table 2. In addition, in the grooves for even passes, θ = 0 degrees. Note that all four angles θ in each groove were the same.

In the present invention, it is determined that the formation of defects is suppressed when the number of defects on the surface of the bloomed rolled material with a depth of 3.0 mm or more is 100 or less per meter in the rolling direction.

Levels 1 to 5 are examples where rolling was performed under conditions that satisfy formula (1).
Specifically, in level 1, θ=7 degrees, and the number of defects was 44.8 defects/m.
Level 2 was θ=15 degrees, and the number of defects was 87.4/m.
At level 3, θ=30 degrees, the number of defects was 95.5/m.
In particular, in level 1, defects in the bulging area were suppressed while defects in the center of the material (the apex of the mountain-shaped structure of the caliber roll with belly, the Z-axis in Figure 4) were reduced, and the number of defects was significantly reduced.

Level 4 was θ=3 degrees, an inclination angle that matched the height of the bulging portion measured in advance by analysis under these rolling conditions, and the number of defects was 39.8 defects/m.
Level 5 was θ=1 degree, and the number of defects was 98.0 defects/m.
In particular, at level 4, the metal flow was strongly controlled to one side, and the occurrence of defects in the corners was accurately suppressed, making it possible to greatly reduce the number of defects.

Levels 6 to 8 are examples where rolling was performed under conditions that did not satisfy formula (1).
Specifically, level 6 is θ = 0 degrees, which is synonymous with the conventional Caliber roll. The number of defects was 114.2/m. Level 7 is θ = 45 degrees, and the number of defects was 235.1/m because the defects on the material center side increased and the material was significantly deformed toward the center up to the vicinity of the bulging part. Level 8 is θ = -10 degrees, and the berry is concave toward the center of the roll. The material to be rolled collapsed during the middle pass, and it was not possible to roll it to the end.

From Table 2, it is clear that by rolling under the conditions of the present invention, the formation of defects can be suppressed.

S: Material to be rolled 1: Rolling mill (splashing mill)
10 Belly caliber roll (grooved rolling roll)
11, 11A, 11B, 11C, 11D Groove portion 12 Side wall portion 13 Bottom portion 100 Caliber roll (grooved rolling roll)
110 Groove 130 Bottom θ, θ ₁ , θ ₂ , θ ₃ , θ ₄ Inclination angle
H Belly height h Caliber depth X1 Flange length

Claims (5)

A caliber roll for rolling a material to be rolled,
The roll has two or more grooves extending in a concave shape in the circumferential direction on the outer surface of the roll, into which the rolled material is fitted during the rolling,
The two or more grooves each have a pair of side wall portions and a bottom portion adjacent to each of the pair of side wall portions as opposed to the surface of the rolled material to be fitted therein,
the bottom portion of at least one of the groove portions has a mountain-shaped structure inclined toward the rolled material side as it is spaced apart from each of the pair of side wall portions,
In the groove portion for odd passes, the inclination angle θ of the mountain-shaped structure with respect to the roll axial direction satisfies the following formula (1),
A caliber roll with a berry, in which the inclination angle θ is 0 degrees in the groove portion for an even number of passes .
0 degrees < θ ≦ 30 degrees ... (1) A method for producing a rolled material by rolling using the caliber roll with berries according to claim 1, comprising the steps of:
In the rolling, the material to be rolled is heated to a heating temperature of 800°C or higher, and then passed between a pair of caliber rolls with bellies, with a rolling reduction rate per pass of 70% or less. The method for manufacturing rolled material according to claim 2, wherein Cr steel containing 5.0 mass% or more of Cr is used as the rolled material. A method for producing steel pipes, comprising the steps of: producing a steel pipe using a rolled material obtained by the method for producing rolled material according to claim 2 or 3; A rolling mill that rolls a material to be rolled,
A pair of caliber rolls with bellies are disposed opposite to each other,
Each of the pair of caliber rolls with berries comprises:
The roll has two or more grooves extending in a concave shape in the circumferential direction on the outer surface of the roll, into which the rolled material is fitted during the rolling,
The two or more grooves each have a pair of side wall portions and a bottom portion adjacent to each of the pair of side wall portions as opposed to the surface of the rolled material to be fitted therein,
the bottom portion of at least one of the groove portions has a mountain-shaped structure inclined toward the rolled material side as it is spaced apart from each of the pair of side wall portions,
In the groove portion for odd passes, the inclination angle θ of the mountain-shaped structure with respect to the roll axial direction satisfies the following formula (1),
the inclination angle θ being 0 degrees in the grooves for an even number of passes .
0 degrees < θ ≦ 30 degrees ... (1)

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