JPH11123408A - Anvil for pressing width of stainless steel slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively reduce the generation of edge seam flow and surface roughening of a stainless steel sheet which is manufactured through a series of hot rolling including edging with a width press and vertical rolls. SOLUTION: The width pressing of a slab M after heating is executed using a convex anvil 1 which is made into 1/15-1/4 of the thickness of the slab in the height KH in the projecting part in the cross section of the convex anvil 1, into 1/3-3/4 of the thickness of the slab in the length KA of the top side of the projecting part into the thickness + (10-30) mm in the length KB of the base. This convex anvil 1 is composed of a inlet-side slope 2 for introducing the hot slab, intermediate parallel surface 3 which is connected to this inlet-side slope 2 and parallel to the transporting line of the hot slab and the outlet-side slope 4 which is connected to the intermediate parallel surface 3 and useful to formation of the rear end part and the pressing surface is formed by combination of these inlet-side slope 2, intermediate parallel surface 3 and outlet-side slope 3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stainless steel slab which can reduce defects (called edge seam flaws) long in the rolling direction near the widthwise edge of the material to be rolled in the process of hot rolling the slab. It relates to an anvil for width press.

[0002]

2. Description of the Related Art As one of means for adjusting the width of a hot slab in a wide range, a width reduction technique using a horizontal counter press has been developed. This method is widely used in hot-rolling mills in Japan and overseas because the contact length at the time of width reduction is large and the rolling efficiency is good, and the molding conditions at the front and rear ends of the slab can be set to minimize the crop. .

[0003] Utilizing anvils used in this horizontal counter press, for the purpose of preventing edge seam flaws, which are one of the surface defects occurring at the edge (width end) of the plate, for example.
Japanese Patent Application Laid-Open No. HEI 9-214572 discloses a technology in which a slab heating temperature is set to 1000 to 1300 ° C., and a width is reduced by an anvil (convex anvil) in which a convex portion is formed at a central portion (a central portion in a thickness direction on a pressed surface of the anvil). A method for preventing slab buckling and reducing edge seam flaws is disclosed in JP-A-5-277510.

[0004] These techniques reduce the wraparound of edge seam flaws by compensating for bulging generated at the end of the slab width in the horizontal rough rolling stage after the width pressing, by making the slab side surface concave by width pressing using a convex anvil. Is what you do. On the other hand, as a technique for reducing surface defects not limited to width pressing, for example, Japanese Patent Application Laid-Open No. 3-124304 has no specific disclosure relating the slab heating temperature to the edge seam flaw, but a slab of martensitic stainless steel. It has been proposed that the heating temperature be 1200 ° C. or more and the heating time be 4 hours or less.

[0005]

By the way, the following problems have been encountered in the prior art for the purpose of preventing edge seam flaws occurring at the widthwise edge portions after hot finish rolling. In the technique disclosed in Japanese Patent Application Laid-Open No. 5-123713, even when the width reduction is small, the convex shape imparted to the anvil is largely transferred to the side surface of the slab because the length of the top of the convex portion is short. For this reason, the depth of the concave portion is increased, and the edge seam flaw on the plate surface can be reduced after the finish rolling, but the concave portion sometimes remains as a defect inside the plate. Further, when the width reduction amount is large, the dent amount on the side surface of the slab is further increased, and the internal defects after finish rolling are increased.

[0006] When the width reduction amount is large, the compressive strain in the width direction also increases in the slab surface layer.
Due to this compressive deformation, the crystal grains on the slab surface became uneven, and the crystal grains on the projections fell down to form edge seam flaws. Note that Japanese Patent Application Laid-Open No. 5-123713 discloses a heating temperature of 10
Although it is described that the temperature is set to 00 to 1300 ° C., there is no description about the heating time. Further, there is no description about the relationship between the heating temperature in this temperature range and the edge seam flaw.

On the other hand, in Japanese Patent Application Laid-Open No. 5-277510, a large width reduction is intended to be achieved by a width press using an anvil provided with a projection at the center of a W-shaped groove. However, even if a large amount of width reduction is obtained, the crystal grains of the surface layer become uneven due to the compression deformation in the width direction of the slab due to the W-shaped groove, and the unevenness of the crystal grains falls down, and remains on the sheet surface until after finish rolling, and the edge seam It was flawed. If the edge seam flaws are large as described above, they remain on the upper and lower surfaces of the sheet until after cold rolling, so that it is difficult to reduce the margin for trimming (trim amount), and the yield has been reduced.

Further, if the heating temperature is set to 1200 ° C. or higher and heating is performed for 4 hours or less as proposed in Japanese Patent Application Laid-Open No. 3-124304, the surface of the decarburized layer on the slab surface can be prevented from being crystallized. Due to the occurrence of edge seam flaws due to grain irregularities, even after cold rolling, the amount of edge seam flaws wrapped around was large, so that the margin for trimming had to be reduced. An object of the present invention is to solve these problems of the prior art at once, and to effectively reduce the occurrence of edge seam flaws and surface roughness of a stainless steel sheet manufactured through a series of hot rolling including width reduction by a width press and a vertical roll. An object of the present invention is to provide an anvil for pressing a stainless steel slab which can be reduced in width.

[0009]

SUMMARY OF THE INVENTION The present invention relates to a convex anvil having a trapezoidal convex portion in the center, wherein the height of the convex portion relating to the cross section of the convex anvil is 1/15 to 1/1 / th of the slab thickness. 4. An anvil for pressing width of stainless steel slab, characterized in that the length of the top of the projection is 1/3 to 3/4 of the slab thickness and the length of the bottom of the projection is the slab thickness + (10 to 30) mm. It is.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 2 is a layout diagram of a row of hot rolling equipment suitable for carrying out the present invention, wherein 10 is a heating furnace, 11 is a width pressing apparatus, 12 is a row rolling mill, and 13 is an edge heater. , 14 is a finishing rolling mill row, 15 is a coiler for winding the steel sheet rolled by the finishing rolling mill row 14, and 16 is a vertical roll rolling installed on each coarse rolling mill entry side and the finishing rolling mill row 14 entering side. Machine (aka edger).

According to the results of the study by the present inventors, the causes of the occurrence of edge seam flaws are as follows. In order to control the width of the material to be rolled, an edger 16 is arranged only on the entrance side of the horizontal rolling mill, or on both the entrance side and the exit side, in the ordinary rolling mill row, and the rough rolling mill row 12 and the finishing rolling mill row are arranged. The structure is such that the roll axis positions of the 14 horizontal rolls and the vertical roll of the edger 16 are shifted by several meters in the rolling direction.

[0012] Therefore, since the vertical roll does not act on the width end of the cross-section where the horizontal roll rolling is performed, the free surface of the width end is bulge-deformed, and at this time, wrinkles are generated in the region. The wrinkles are gradually increased by a plurality of passes by the downstream edger 16, and eventually reach the surface of the material to be rolled to form edge seam flaws. The main cause of the wrinkles is due to deformation and fall of coarse crystal grains on the surface. That is, the surface layer of the continuously cast stainless steel slab is decarburized when heated in a heating furnace, the crystal grains of the surface layer of the slab are coarsened, and the crystal grains are convex due to the compressive deformation in the width direction when the width is reduced. (The crystal grains protrude from the surface layer), and the protruding crystal grains fall down in the rough rolling and the finish rolling to form edge seam flaws.

[0013] In particular, the amount of decarburization of ferritic stainless steel having a crystal structure of ferrite is larger than that of other stainless steels, and the decarburized layer has a thickness of about 1 mm to several millimeters.
In addition, since the range of unevenness in the width press is wide, the amount of wrap around edge seam flaws is large. The temperature regulation effective for suppressing the edge seam flaws is to reduce the heating temperature of the slab and regulate the heating time, and further regulate the temperature at the width end of the sheet bar before the finish rolling. The former prevents crystal grain coarsening due to decarburization of the slab surface layer, and the latter prevents roughening of the skin.

By lowering the heating temperature, the thickness of the decarburized layer becomes thinner, the irregularities of the crystal grains in the compression deformation in the width direction at the time of reducing the width become smaller, and the depth becomes shallow so as not to fall down, and the temperature becomes lower. Edge seam flaws disappear after cold rolling. The reason is that when the heating temperature is lowered to a certain level,
1mm on the slab surface, which tends to develop edge seam flaws later
This is because such coarse crystal grains disappear.

Crystal grains are lined up in layers on the slab surface, and these grains are most strongly decarburized during heating. In order to find an effective means to reduce the thickness of this decarburized layer to 1 mm or less, a 200 mm thick slab is heated, and the heating temperature, heating time, and oxygen partial pressure are variously changed. The thickness of the decarburized layer on the surface of the slab was investigated. Figure 3 shows the resulting decarburized layer thickness of the slab surface and the heating temperature, heating time,
4 is a graph illustrating a relationship with an oxygen partial pressure.

As shown in FIG. 3 (a), the thickness of the decarburized layer has a large correlation with the heating temperature, and increases with the heating temperature. It reaches more than 4 mm at 1300 ° C. Although the thickness of the decarburized layer is increased by the oxygen partial pressure as illustrated in FIG. 3B, the rate of increase is small. Speaking of ease of rolling, it is desirable to suppress the thickness of the decarburized layer by raising the heating temperature and lowering the oxygen partial pressure, but in actual operation, there is a limit to the sealing during slab insertion and extraction, and the oxygen partial pressure is reduced to 1 %, It is difficult to reduce the decarburized layer thickness, so that the heating temperature should be lowered to such an extent as not to hinder the rolling in order to reduce the thickness of the decarburized layer.

Based on the results of the investigation, it has been found that the heating temperature is preferably set to 1100 ° C. or less in order to stably reduce the slab decarburized layer thickness to 1 mm or less in consideration of variations in actual operation. However, if the heating temperature is lower than 900 ° C., the rolling becomes difficult. Therefore, the heating temperature must be 900 ° C. or higher. As for the heating time, for example, as shown in FIG. 3C, the longer the heating time, the thicker the decarburized layer tends to be, but the increase rate is not so large. However, when the heating temperature is at the upper limit of the preferred range (1100 ° C.), if the heating time exceeds 6 hours, the thickness of the decarburized layer becomes 1 mm or more, so the heating time should be 6 hours or less. desirable.

Considering uniform heating up to the center of the slab, for example, when the heating temperature is at the lower limit of 900.degree. On the other hand, if the heating temperature is regulated to 900 to 1100 ° C. and the heating time is restricted to 6 hours or less as described above, the temperature at the width end of the material to be rolled in finish rolling tends to decrease to less than 800 ° C. In this case, the rolling load increases, the black scale on the surface of the roll peels off, causing considerable unevenness on the roll and causing roughening of the plate.
14 On the inlet side, set the width end temperature to 80
It must be kept above 0 ° C.

When the temperature exceeds 900 ° C., the generation and growth of the black scale on the roll surface layer exceeds the peeling, and the roll surface layer is uniformly covered with the black scale. It is even more preferred to maintain the temperature at or above ° C. In addition, the temperature regulation described above is particularly effective when applied to a ferritic stainless steel sheet in which the decarburization amount is relatively large and the crystal grains of the surface layer tend to be coarse, but the effect is remarkably exhibited. It is effective even if applied.

Next, the present invention will be described. 1A and 1B are a perspective view, an external perspective view, a cross-sectional view of a convex part, and a plan layout view, respectively, of a convex anvil according to the present invention. Anvil 2, an entrance side inclined surface for introducing a hot slab, 3 is an intermediate parallel surface connected to the entrance side inclined surface 2 and parallel to the hot slab conveyance line, and 4 is connected to an intermediate parallel surface 3 and a rear end of the slab. A press-formed surface is formed by a combination of the entrance-side inclined surface 2, the intermediate parallel surface 3, and the exit-side inclined surface 4, which is an exit-side inclined surface useful for part forming. Reference numeral 5 denotes a top of the convex portion, 6 denotes an inclined portion of the convex portion, 7 denotes a bottom portion of the convex portion, and KH, KA, and KB denote a convex portion height, a convex portion top side length, and a cross section of the convex anvil 1, respectively. M represents a slab (hot slab).

As shown in FIG. 1 (c), the convex anvils 1 form a pair, and are arranged so as to sandwich the hot slab M conveyed in the direction of the left arrow from both sides. By repeating the reciprocating movement of each other (the driving means is not shown), the slab M being conveyed is reduced in width over its entire length. As described at the beginning, the width reduction in such a form is referred to as “horizontal counter press”.

In order to make it easy to bite into the side surface of the slab, the convex portion of the metal anvil 1 has a “trapezoidal shape”, that is, a shape in which the interval between the substantially symmetrically facing inclined portions 6 becomes narrower toward the top portion 5. The present invention is to suppress the occurrence of edge seam flaws when the width reduction of a stainless steel slab is performed by a width press using a horizontal opposing press anvil under the above temperature regulation, and a convex anvil having a specific cross-sectional dimension is used. By forming the slab side into an appropriate concave shape by adopting
By compensating for bulging of the slab side surface during horizontal rolling, preventing wrinkles generated on the slab side surface from wrapping around the front and back, and reducing the compressive force applied to the surface layer, reducing the irregularities of crystal grains generated on the surface However, the fall of the crystal grains can be prevented, and the occurrence of the edge seam flaw can be limited to only the extreme edge portion of the width.

Here, the present inventors have shown in experiments that when the thickness is reduced by 50% or more in the first three passes of the rough rolling, the cross-sectional shape at the width end of the material to be rolled after the completion of the third pass of the rough rolling is rectangular. It is found that there is a tendency that the wraparound amount of the side surface in the downstream rough rolling and finish rolling tends to be smaller as the distance is closer to the slab. The preferred range of the cross-sectional dimension of the convex anvil was determined so that a cross-section at a width end near a rectangle could be obtained by compensation by bulging by horizontal rolling.

For this reason, after the width pressing with the anvil for width pressing of the present invention, when the width reduction by the vertical roll intervenes in the course of the first to third passes of the rough rolling, the width end of the material to be rolled on the entry side in the fourth pass is reduced. Instead of a rectangular shape, the amount of side wrap around in downstream rough rolling and finish rolling becomes large. Therefore, the width was reduced to 50% or more without performing width rolling with vertical rolls in the first three passes of rough rolling after width pressing.

The reasons for limiting the cross-sectional dimensions of the anvil will be described below. [Protrusion height (KH)] A ferrite stainless steel slab with a slab thickness of 200 to 260 mm is heated to 1000 ° C, and a width press is performed by using a flat anvil with a flat pressed surface while changing the width press amount variously. Subsequently, a width press experiment was performed in which the rough rolling was performed three times without vertical rolling using a vertical roll to reduce the total rolling reduction to 50%, and the bulging amount after the rough rolling was examined.

Here, in the present invention, the "width press amount" means a difference in slab width before and after the width press, and the "bulging amount" means a slab side end corner portion before the width press is subjected to rough horizontal rolling. It means the distance from the position moved to the plate surface after the pass to the end of the slab width after the width press. FIG. 4 is an explanatory diagram of the bulging amount. FIG. 4 shows a cross section of the width end portion of the slab after the width pressing. P _SC is a slab side end corner portion before the width pressing, and V is a bulging amount.

FIG. 5 is a graph showing the relationship between the width press amount and the bulging amount to the slab thickness ratio (V / H) obtained by arranging the width press-rough rolling experiment results. is there. As shown in FIG. 5, V / H is the width pressing amount of 50 mm.
1/15 to 1/4 with the increase from about 200 mm to about 200 mm
To increase. Therefore, by providing a dent of a depth corresponding to the slab side surface in advance, rough horizontal rolling 3
The bulging amount after the pass is compensated, and the end of the cross-sectional width of the material to be rolled becomes almost rectangular. Therefore, it is preferable that the height KH of the convex anvil for providing such a depression is 1/15 to 1/4 of the slab thickness. [Convex Top Length (KA)] In the above-described width press experiment, the convex height KH was set to the above-mentioned preferred range in place of the flat anvil, and the convex top length KA and the convex bottom length KB were variously changed. The bulging amount after three passes of rough horizontal rolling (no width reduction by vertical rolls) was examined.

FIG. 6 is a graph showing the relationship between the resulting slab thickness ratio (KA / H) and the cross-sectional shape of the material to be rolled and the bulging amount (V). As shown in FIG. 6, when KA / H is less than 1/3, the cross-sectional shape becomes double bulging, and V increases as KA / H decreases. If KA / H exceeds 3/4, the cross-sectional shape becomes single bulging, and V increases as KA / H increases. When KA / H is in the range of 1/3 to 3/4, the cross section has a shape close to a rectangle intermediate between double bulging and single bulging, and V is the lowest value in this range.

FIG. 6 illustrates a case in which the slab thickness is 200 mm, the height of the convex portion of the convex anvil KH is 18 mm, the length of the convex portion base is KB 220 mm, and the width of the pressed portion is 100 mm. Is similarly observed over the preferred range of KH and KB according to the present invention and the range of the slab thickness and width press amount shown in FIG. Therefore, the length KA of the top of the convex portion is 1 of the slab thickness.
/ 3 to 3/4 is preferred. [Protrusion Bottom Length (KB)] FIG. 7 shows the transfer rate and the excess (KB) of the protrusion bottom length to the slab thickness in the above-described width pressing experiment, which were examined for the slab after the width pressing and before the rough rolling.
-H) is a graph showing the relationship. Here, the “transfer rate” is a percentage (%) of the maximum dent depth of the slab side surface with respect to the convex portion height KH, and 100% is a state where the convex portion height KH is completely transferred to the slab side surface. is there.

As shown in FIG. 7, when the KB-H is less than 10 mm and the transfer rate exceeds 90%, the slab side corners are not formed at the bottom of the anvil, but at the bottom at the time of width pressing. Since it is pressed by the flat portion, the compressive deformation in the width direction increases, and the unevenness of the crystal grains on the surface layer of the slab also increases, which tends to remain as edge seam flaws. On the other hand, if KB-H exceeds 30 mm, the transfer rate is less than 80%, and the dent depth of the side surface of the slab is insufficient, so that the cross section of the material to be rolled after three passes of rough rolling cannot be made close to a rectangle.

FIG. 7 shows an example in which KH is H / 10, the width pressing amount is 200 to 220 mm, and KA / H is 1/3 and 3/4. The tendency shown in FIG. The same was observed over the preferred range of KH according to the present invention and the range of the slab thickness and width pressing amount shown in FIG. Therefore, it is preferable that the length KB of the bottom of the convex portion is set to the slab thickness H + (10 to 30) mm.

[0032]

[Example] Slab: C: 0.05wt%, Si: 0.3wt%, M
n: Continuous cast thickness 200m containing 0.1wt%, Cr: 17wt%
This is a ferrite stainless steel slab having a length of 1300 mm, a width of 1300 mm, and a length of 6 m, and was subjected to hot rolling using the row of hot rolling equipment shown in FIG. The work roll diameter of the rough rolling mill is 1300
mm, the barrel length is 2200 mm, the work roll diameter of the finishing mill is 700 mm, the barrel length is 2000 mm, and the exit speed of the finishing mill row 14 is 1000 m / min.

As Invention Examples B1 to B6, after the slab was heated in a heating furnace under the conditions of 1000 ° C. × 4 hours, a width press amount of 50 mm was obtained using three types of convex anvils within the range of the sectional dimensions of the present invention. A 200 mm width press was performed, and then the first three passes were reduced to a thickness of 100 mm (thickness reduction rate: 50%) by a row of four-stand rough rolling mills without reducing the width by vertical rolls. And a total of 6 passes of rough rolling are performed to form a sheet bar having a thickness of 25 mm and a width of 1300 mm on the exit side of the row of coarse rolling mills. While maintaining the temperature at 800 ° C. or higher, the rolls were finished and rolled to a thickness of 4 mm by a row of finishing stands of seven stands. Further, these hot-rolled coils were cold-rolled at a rolling reduction of 70% by a four-stand cold rolling mill after pickling.

On the other hand, as Comparative Examples B7 to B10, the slab was heated in a heating furnace at 1200 ° C. for 4 hours.
No. 3713, a convex anvil having the dimensions described in
Using one type of anvil having a convex portion at the center of the W-shaped groove described in Japanese Patent No. 277510, width pressing was performed with a width press amount of 50 mm and 200 mm. In the same process as in the invention, except that the heating by the edge heater was not performed, the sample was finished rolled to a thickness of 4 mm and wound up. In addition, cold rolling was performed in the same manner as in the invention.

Table 1 shows the cross-sectional dimensions (convex height KH, convex top length KA, convex bottom length KB) of the convex anvil of the invention example and the convex anvil and the W-groove anvil of the comparative example. As shown, the depth of the W-shaped anvil was 300 mm. In addition, Comparative Examples B9 and B10 in Table 1 use anvils with W-shaped grooves.
Table 1 shows the seam amount after hot rolling for the middle portion in the longitudinal direction of the hot rolled coil thus obtained. Here, the seam amount is an evaluation value of an edge seam flaw employed in the present invention,
It means the distance from the width end to the flaw closest to the center of the width (including both edge seam flaws and rough skin), and generally corresponds to the trim amount. In addition, the seam amount is determined in the width direction OP (operator) side, DR (drive) side, and upper and lower surfaces ("OP" in the table).
Under, DR under, etc.).

The seams after hot rolling of Invention Examples B1 to B6 show better results than those of Comparative Examples. In Comparative Examples B7 and B8, the slab width side edge directly hit the bottom of the anvil protrusion,
Since the slab surface layer was strongly compressed in the width direction, the surface layer crystal grains became uneven, and the amount of hot-rolled seam increased. Also, the larger the width press amount, the larger the seam amount. Comparative Examples B9 and B
In Example 10, the restraining force by the W-shaped groove acted, and the slab surface layer was further strongly compressed in the width direction, thereby increasing the degree of unevenness of the surface layer crystal grains.
It became the largest hot rolled seam.

When the seam amount after cold rolling was examined on the upper and lower surfaces of the middle portion in the longitudinal direction of the cold-rolled coil in the same manner as described above, the seam amount after cold rolling was almost the same value as the seam amount after hot rolling.

[0038]

[Table 1]

[0039]

According to the present invention, the edge seam flaws of a stainless steel plate produced by a series of hot rolling including width reduction by a width press and a vertical roll can be further reduced than before and the surface roughness can be suppressed. This has a remarkable effect that the trim amount of the stainless steel plate, which was conventionally 30 mm / one side, can be reduced to 10 mmm / one side or less.

[Brief description of the drawings]

FIG. 1 relates to the shape and arrangement of anvils according to the present invention;
(A) is an external perspective view, (b) is a sectional view of a convex portion, and (c) is a plan layout view.

FIG. 2 is a layout diagram of a row of hot rolling equipment suitable for carrying out the present invention.

FIG. 3 is a graph illustrating the relationship among the decarburized layer thickness of the slab surface layer, the heating temperature, the heating time, and the oxygen partial pressure.

FIG. 4 is an explanatory diagram of a bulging amount.

FIG. 5 is a graph showing a relationship between a width press amount by a flat anvil and a slab thickness ratio (V / H) of a bulging amount.

FIG. 6 is a graph showing a relationship between a ratio of a top side length of a convex portion to a slab thickness (KA / H), a cross-sectional shape of a material to be rolled, and a bulging amount (V).

FIG. 7 is a graph showing the relationship between the transfer rate and the excess (KB-H) of the length of the base of the convex portion to the slab thickness.

[Description of Signs] 1 convex anvil 2 entrance side inclined surface 3 intermediate parallel surface 4 exit side inclined surface 5 top of convex portion 6 inclined portion of convex portion 7 bottom of convex portion 10 heating furnace 11 width pressing device 12 coarse rolling mill train 13 edge heater 14 finish rolling mill train 15 coiler 16 vertical rolling mill (edger) H slab thickness KH convex height KA protrusion top side length KB protrusion base length M slab (hot slab) P _SC width press Front slab side end corner V Bulging amount

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Kunio Isobe 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Technical Research Institute of Kawasaki Steel Corporation (72) Inventor Masao Yarida 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Steel Research Institute

Claims (1)

[Claims] 1. A convex anvil having a trapezoidal convex portion at a central portion, wherein a height of the convex portion relating to a cross section of the convex anvil is set to 1 of a slab thickness.
/ 15 to 1/4, the length of the convex top is 1/3 to 3 / of the slab thickness
4. An anvil for width pressing of stainless steel slabs, characterized in that the base length of the projections is slab thickness + (10 to 30) mm.