JP5447702B2 - Production method of direct quenching type thin wall steel plate - Google Patents

Description

  The present invention relates to a method for producing a thin-walled steel plate using a through-type cooling device, and more particularly to a method capable of producing a thin-walled steel plate having a long plate length that is excellent in homogeneity of material properties in the longitudinal direction of the steel plate by direct quenching. .

  As a method for producing high-tensile steel having a tensile strength of 600 MPa or more, production by quenching and tempering is generally used. In the past, after rolling to a predetermined thickness by hot rolling, the steel sheet cooled to room temperature was reheated and quenched, but in recent years there is a direct quenching-tempering process in which quenching is performed immediately after hot rolling. It is used.

  However, since the direct quenching-tempering process is an on-line heat treatment using a pass-through cooling device, when the plate thickness is thin or the plate length is large, a large quenching start temperature difference occurs between the front end and the tail end of the steel plate.

  That is, when the rolling speed is low in quenching cooling or when the plate length of the steel plate reaches several tens of meters, it is difficult to start cooling all the steel plates at the same time. Therefore, a time difference until the start of cooling occurs, and as a result, a temperature difference at the start of cooling occurs.

  The cooling start temperature during quenching affects the microstructure, and it is difficult to obtain a homogeneous material in the longitudinal direction of the steel sheet. It is desirable to limit the plate length according to the plate thickness so that the fluctuation range of the material falls within a predetermined range, or to manufacture by a reheating quenching-tempering process, which decreases manufacturing efficiency and increases manufacturing cost. Absent. For this reason, various methods for reducing the variation in material have been proposed.

  For example, Patent Document 1 proposes to manufacture a steel material with little change in material by controlling the cooling rate, and describes that the thickness direction of the steel plate and the homogeneity between steel materials are improved. However, in the invention described in Patent Document 1, the material change cannot be suppressed when the cooling start temperature or the finishing temperature of rolling changes.

  In addition, Patent Document 2 describes that the temperature decrease in the four circumferences of the steel material is suppressed by a prior cooling method, and the entire material is rolled in a uniform temperature state, although the rolling finishing temperature can be made constant. As for the subsequent cooling start temperature, there is a difference at the leading and trailing ends of the plate length, so there is a problem that the application with the quenching material and the accelerated cooling material cannot increase the homogeneity.

  Furthermore, although the technique disclosed in Patent Document 3 is effective in suppressing variations among steel materials, it is not sufficient to make the material in the plate length direction in the same steel plate uniform.

  As described above, in the technology disclosed heretofore, since the temperature change in the longitudinal direction of the steel sheet cannot be controlled, the cooling start temperature at the time of cooling cannot be kept constant.

JP 9-310117 A JP-A-3-173716 Japanese Patent No. 3784265

  As described above, in the direct quenching-tempering process using the pass-through quenching apparatus, it is difficult to eliminate the cooling start temperature difference in the longitudinal direction of the steel sheet, the strength and toughness vary greatly, and the thickness and length of the steel sheet are limited. Or, the reheating quenching-tempering process forced high-cost manufacturing.

  Therefore, the present invention is a direct quenching-tempering process in which the tensile strength (TS) is ± 25 MPa at the tip and tail ends of the steel sheet, and the ductility-brittle fracture surface transition temperature (vTrs), which is an indicator of toughness, is ± 10 ° C. An object of the present invention is to provide a direct-quenching thin-walled steel plate having excellent strength and toughness uniformity in the longitudinal direction of the steel plate and a method for producing the same. The direct quenching type refers to those that are manufactured by tempering after direct quenching.

  As a means for solving the above-mentioned problems, the inventors have arranged a cooling device with a short cooling region on the downstream side in the vicinity of the hot rolling mill to give an appropriate temperature gradient in the longitudinal direction of the steel plate before subsequent quenching. As a result, it has been found that a stable material can be obtained in the longitudinal direction of the steel plate even in actual operation.

The present invention was made by further study based on the obtained knowledge, that is, the present invention is
1. A direct quenching type thin steel plate having the following steps using a rolling-cooling device in which a first passing type cooling device and a second passing type cooling device are arranged in order on the downstream side of a reversible hot rolling mill. Production method.
(1) A step of applying a temperature gradient in the longitudinal direction of the steel sheet with the first passage type cooling device before cooling the steel plate after finish rolling with the second passage type cooling device.
(2) A step of quenching the steel sheet by passing a second passage type cooling device at a constant speed.
2. When cooling with the first passage type cooling device, the temperature difference ΔT between the steel plate front end and the tail end after cooling is cooled so as to satisfy the expression (1) by changing the conveyance speed and / or the amount of water injection. 2. The method for producing a direct-quenching thin-walled steel plate according to 1, wherein
15L / (hv) ≦ ΔT ≦ 55L / (hv) (1)
However, h: board thickness (mm), L: length (m), v: approach speed (m / s) to 2nd passage type cooling device
3. When quenching with the second passage type cooling device in the step 2, the steel plate length in the step 1 is set so that the cooling start temperature at the tip and tail ends of the steel plate becomes Ar3 point or higher or a two-phase region temperature. The method for producing a direct-quenching thin-walled steel sheet according to 1 or 2, wherein the temperature at the tip end and the temperature at the tail end in the direction are adjusted.
4). When quenching with the second passage type cooling device in the step 2, the tip in the longitudinal direction of the steel plate in the step 1 so that the difference in cooling start temperature between the tip and the tail end of the steel plate is within 50 ° C. The method for producing a direct-quenching thin-walled steel sheet according to any one of 1 to 3, wherein the temperature of the head portion and the temperature of the tail end portion are adjusted.
5. The composition of the steel sheet is mass%, C: 0.01 to 0.20%, Si: 0.01 to 0.80%, Mn: 0.50 to 2.50%, P: 0.020%. Hereinafter, S: 0.0070% or less, sol. The production of a direct quenching type thin-walled steel plate according to any one of 1 to 4, wherein Al: 0.004 to 0.100% is contained, and the balance is composed of Fe and inevitable impurities Method.
6). Further, as a component composition, steel: Ti: 0.005-0.20%, Cu: 0.01-2.0%, Ni: 0.01-4.0%, Cr: 0.01 -2.0%, Mo: 0.01-2.0%, Nb: 0.003-0.1%, V: 0.003-0.5%, W: 0.003-0.7%, One or more of B: 0.0005-0.0040%, Ca: 0.0001-0.0060%, Mg: 0.0001-0.0060%, REM: 0.0001-0.0200% 5. The method for producing a direct-quenching thin-walled steel plate according to 5, wherein:

  In the present invention, the steel sheet temperature such as the rolling finish temperature, the cooling start temperature, and the cooling stop temperature refers to a temperature obtained by measuring the temperature of the steel sheet surface with a radiation thermometer unless otherwise specified.

  Moreover, in this invention, a front-end | tip part and a front-end | tip shall refer to the area | region containing the front-end | tip of the stationary part used for a product, all near the front-end | tip of the advancing direction of a steel plate, without distinguishing each other. Similarly, the tail end and the tail end are both in the vicinity of the rear end in the traveling direction of the steel sheet and are not distinguished from each other, and indicate a region including the rear end of the steady portion used in the product.

  According to the present invention, it is possible to make the cooling start temperature constant during quenching in the longitudinal direction of the steel sheet by direct quenching treatment using a plate-type quenching apparatus for a steel sheet having a thickness of 6 mm to 25 mm and a plate length of 20 m to 50 m. Thus, a high-strength and high-toughness steel having excellent longitudinal uniformity of the steel sheet can be obtained, which is extremely useful industrially.

The schematic diagram which shows an example of rolling-cooling equipment. The schematic diagram explaining the rolling-cooling method which concerns on this invention. The schematic diagram explaining the temperature difference of the steel plate longitudinal direction in the rolling-cooling method which concerns on this invention. In the present invention, a cooling device excellent in draining performance suitable as a first-pass type cooling device (a) does not use a draining roll, and a cooling water jet nozzle is used to retain cooling water on a steel plate, (b) is cooling. A form in which the cooling water is retained on the steel sheet by using the water injection nozzle and the draining roll together, and (c) is a form in which the cooling water is retained on the steel sheet by the draining roll without using the cooling water injection nozzle. The figure which shows the cooling rate at the time of air cooling of a hot-rolled steel plate.

In the present invention, when the steel sheet after finish rolling is cooled by a pass-through type direct quenching device, the cooling start temperature is lowered from the tip side on the steel plate tail end side, before the steel plate is directly carried into the quenching device. The problem is solved by compensating the temperature on the tail end side of the steel plate by the amount of the drop.
Hereinafter, the present invention will be described in detail with reference to the drawings.

  FIG. 1 schematically shows the arrangement of rolling mills and cooling devices in a rolling-cooling facility to which the present invention is applied. In the figure, 12 is a heating furnace, 9 is a rolling mill, and 10 is arranged downstream of the rolling mill 9. In the first passage type cooling device, when (10) is arranged on the upstream side of the rolling mill 9, 11 denotes a second passage type cooling device. The heating furnace 12 side of the rolling mill 9 is referred to as the upstream side of the rolling mill 9, and the second passage type cooling device 11 side is referred to as the downstream side.

  A thick steel plate (not shown) is heated for rolling in the heating furnace 12, and after finish rolling, an appropriate temperature gradient is applied in the longitudinal direction of the steel plate by the first passage type cooling device 10.

  The steel plate after the finish rolling is transferred to the second passage type water cooling device 11 for quenching, but the first passage type cooling device 10 is previously set so that the cooling start temperatures are substantially the same in the longitudinal direction of the steel plate. When passing, an appropriate temperature gradient is given in the longitudinal direction of the steel sheet.

  FIG. 2 is a schematic diagram for explaining a rolling-cooling method in the steel sheet manufacturing method according to the present invention, P1 to P4 indicate paths, P3 is a final (finishing) path, P2 is a path immediately before the final path P3, P1 is a pass immediately before P2, and P4 is an empty pass for passing the steel sheet after finish rolling or finish rolling and water cooling through the rolling mill 9.

  a is a rolling operation by the rolling mill 9, b1 and b2 are water cooling operations by the first passage type cooling device 10, and c is a water cooling operation by the second passage type cooling device 11. Also in the figure, the steel plate is not shown.

  FIG. 2A shows a case where the first passage type cooling device 10 and the second passage type cooling device 11 are arranged in this order on the downstream side of the rolling mill 9, and the steel plates after finish rolling are sequentially passed and cooled (Case 1). In FIG. 2 (b), the first passage type cooling device 10 is arranged upstream of the rolling mill 9, the second passage type cooling device 11 is arranged downstream of the rolling mill 9, and the steel plate after finish rolling (P3) is empty. When cooled by the first passage type cooling device 10 in the pass (P4) and then passed through the rolling mill 9 by the empty pass (P4) and cooled by the second passage type cooling device 11 (Case 2), FIG. 2 (c) Is a case where a passage-type cooling device (first passage-type cooling device 10 in the present description) is arranged downstream of the rolling mill 9 and cooled at one end, and then cooled by the passage-type cooling device while being fed back ( Case 3) is shown.

  FIG. 3 schematically shows the temperature distribution in the longitudinal direction of the steel sheet in Cases 1 to 3 shown in FIG. In the drawing, it is assumed that the steel plate proceeds in the direction of the arrow, and in the description, the traveling direction side of the steel plate is referred to as the front end portion of the steel plate, and the opposite direction is referred to as the tail end portion of the steel plate. Hereinafter, the present invention will be described with reference to FIGS.

  In the case 1, the steel sheet is subjected to the final pass P3, and after having a predetermined thickness, the steel sheet is water-cooled b1 by the first passage type cooling device 10 and then water-cooled c by the second passage type cooling device 11, and the desired performance is obtained. Is granted.

  The water cooling b1 is a temperature at which the tail end portion becomes higher than the tip end portion by ΔT in the longitudinal direction of the steel plate so that the cooling start temperature becomes substantially the same in the longitudinal direction of the steel plate when the water cooling is performed by the second passage type cooling device 11. Give a gradient.

  When water cooling c is performed by the second passage type cooling device 11, the steel sheet is allowed to enter the second passage type cooling device 11 at a constant speed, and the cooling stop temperature is made substantially the same in the longitudinal direction of the steel sheet.

  In Case 2, the steel sheet is made to have a predetermined thickness by the rolling mill 9 in the final pass P3, then water-cooled b1 by the first passage type cooling device 10, and then passes through the rolling mill 9 by the empty pass P4. Water cooling is performed by the second passage type cooling device 11, and desired performance is imparted.

  When the water-cooling b1 is water-cooled by the second passage type cooling device 11, the tail end is set to a temperature higher than the tip by ΔT in the longitudinal direction of the steel plate so that the cooling start temperature is substantially the same in the longitudinal direction of the steel plate. When water cooling c is performed by the second passage type cooling device 11, the steel sheet is allowed to enter the second passage type cooling device 11 at a constant speed, and the cooling stop temperature is made substantially the same in the longitudinal direction of the steel sheet.

  In Cases 1 and 2, by allowing the steel sheet to pass through the first passage type cooling device 10 while accelerating, the tail end portion can be heated at a temperature higher by ΔT than the front end portion in the longitudinal direction of the steel plate.

  In Case 3, the steel plate is subjected to a final pass P3, and after having a predetermined plate thickness, the water cooling b1 is performed by the first passage cooling device 10 and then the water cooling b2 is performed again while the first passage cooling device 10 is fed back. .

  In the water cooling b1, the tip is heated at a higher temperature by ΔT than the tail end in the longitudinal direction of the steel sheet. When water cooling b1 is performed, the steel sheet can be heated at a temperature higher than the tail end by ΔT by passing the first passing cooling device 10 while decelerating.

  When water cooling b2 is performed while feeding back, the tail end portion becomes the tip portion, and the tip end portion becomes the tail end portion, so that the cooling start temperature is substantially the same in the longitudinal direction of the steel sheet.

  When water cooling b2 is performed by the first passage type cooling device 10 while being reversely fed, the steel plate enters the first passage type cooling device 10 at a constant speed, and the cooling stop temperature is made substantially the same in the longitudinal direction of the steel plate. Reverse feed refers to the case where the steel sheet is transported from the downstream side of the rolling mill 9 to the upstream side.

  In Cases 1 to 3, the temperature difference ΔT applied to the tail end and the tip end in the longitudinal direction of the steel plate is the steel plate in the subsequent cooling by the second passage type cooling device 11 or the first passage type cooling device 10 performed in the reverse direction. The cooling start temperature at the tail end is applied so as to be the same temperature as the tip.

  As shown in FIG. 3, it is preferable to change the temperature gradient linearly in the longitudinal direction of the steel sheet, but the temperature gradient may be changed stepwise in the longitudinal direction of the steel sheet.

  Here, the solid line in FIG. 5 shows an example in which the relationship between the thickness of the hot-rolled steel plate at 700 ° C., 850 ° C. and 1000 ° C. and the cooling rate when the steel plate is cooled by air cooling is obtained by heat transfer calculation. It is a thing. The solid line is the calculation result. The smaller the plate thickness, the smaller the heat capacity, and the higher the steel plate temperature, the more radiation and heat radiation, so the cooling rate increases.

  Although the cooling rate differs somewhat depending on the form of transport and atmosphere, for example, the cooling rate when the plate thickness is 30 mm and the steel plate surface temperature is 800 ° C. is about 0.8 ° C./s.

  Generally, after cooling with the first passage type cooling device, the start temperature of accelerated cooling or quenching when the second passage type cooling device or the first cooling facility again cools is 700 ° C. or higher, The cooling rate is not less than the broken line of 15 / h (° C./mm) in FIG.

On the other hand, since the start temperature of accelerated cooling or quenching hardly exceeds 1000 ° C., the cooling rate is similarly below the broken line of 55 / h (° C./mm).
Therefore, when the accelerated cooling or quenching start temperature is 700 ° C. or more and 1000 ° C. or less, the cooling rate is in the range of 15 / h or more and 55 / h or less, and therefore the temperature difference ΔT at the leading end of the temperature gradient applied in the longitudinal direction. Is
15L / (hv) ≦ ΔT ≦ 55L / (hv) (1)
It is preferable to satisfy this relationship. However, h (mm): the thickness of the steel plate, L (m): the length of the steel plate, v (m / s): a temperature gradient in the longitudinal direction of the steel plate by the first passage type cooling equipment, and then performed. The steel sheet transport speed when entering the cooling facility.

  As an example, when the length of the steel sheet product is L = 30 m and the conveyance speed is v = 1 m / s, the cooling start is different at the tip and the tail, so the tail end is different from the tip. Air-cooled for an additional 30 s.

  When the plate thickness is h = 30 mm, the cooling rate when the steel plate surface temperature is 800 ° C. is about 0.8 ° C./s, so that the tail end portion is 0.8 × L / v = 24 ° C. than the tip end portion. Lower. Therefore, if the tip is cooled by 34 ° C. and the tail end is cooled by 10 ° C., the temperature difference at the tail end is 24 ° C. higher than the tip, so that the cooling start temperature is substantially constant.

  In addition, it is preferable to use the cooling device excellent in the draining property which does not flow out cooling water in the steel plate conveyance direction other than a cooling area as a 1st or 3rd passage type cooling device.

  4 (a), 4 (b), and 4 (c) show an example of a cooling device excellent in draining performance, in which 1 is a cooling tank, 2 is cooling water staying on the steel plate 4, and 3 is upper cooling water. The injection nozzle, 4 is a steel plate, 5 is a transport roll, 6 is a lower cooling water nozzle, 7 is a cooling region, 13 is a rod-shaped cooling water, and 8 is a draining roll.

(A) is a cooling device which ejects the rod-shaped cooling water 13 from the upper cooling water injection nozzle 3 attached to the cooling tank 1 so as to oppose, and retains the cooling water 2 on the steel plate 4, and (b) is the draining roll 8 And the transport roll 5 are opposed to each other with the steel plate 4 interposed therebetween, and a cooling device that ejects the rod-shaped cooling water 13 from one direction and stops it with the draining roll 8, (c) is a pair of the draining roll 8 and the transport roll 5, The cooling device which makes the cooling water 2 stay on the steel plate 4 is shown. In order to obtain a damming effect, it is preferable that the rod-shaped cooling water 13 has a water density of 4 m ³ / m ² min or more.

  It is preferable that the distance in the conveyance direction of the cooling region is 0.4 m to 4 m in the first passing type cooling device (including the case 3 passing type cooling device). If it is less than 0.4 m, it is necessary to take a long residence time in the cooling region in order to cool the steel sheet, and it takes too much time to pass through the entire steel sheet, making it difficult to provide a sufficient temperature gradient.

  On the other hand, if it exceeds 4 m, it is difficult to provide uniform cooling in the cooling region, and it is difficult to provide a sufficient temperature gradient with a steel plate with a short plate length, so it is limited to 4 m or less. .

  In addition, the cooling area | region 7 in FIG. 4 (a), (b), (c) is shown by the black coating part of a steel plate. The cooling region is appropriately set by increasing / decreasing the number of cooling tanks and nozzles, or by increasing / decreasing the number of cooling units shown in FIGS. 4 (a), (b), and (c). It is possible.

  In order to provide a temperature gradient with the first passing type cooling device, it is preferable to control the passing speed of the steel sheet in the cooling region because of excellent control responsiveness. The cooling facility capacity such as the water injection amount of the first passage type cooling device may be controlled, and both the passage speed and the cooling capacity may be used.

  The second passage type cooling device is not particularly limited as long as it has a required cooling capacity and can be uniformly cooled.

As the cooling start temperature, an Ar ₃ point or more and a two-phase region temperature are appropriately selected according to desired characteristics.

This is because, in order to secure the desired strength by generating a transformation phase such as martensite or bainite by quenching or accelerated cooling from the temperature range including the austenite phase, the cooling start temperature of quenching or accelerated cooling is higher than the Ar ₃ transformation point. Or it is because it must be a temperature range in which the austenite phase exists, that is, a two-phase temperature range, but a specific cooling start temperature may be appropriately selected according to a desired strength.

In addition, it is preferable to temper the hardened steel plate. Tempering may be carried out by a conventional method. For example, an off-line atmosphere furnace or an on-line induction heating device can be used, and the tempering temperature is below the Ac ₁ transformation point, which is a temperature range in which an austenite phase is not generated. It is preferable that it is the temperature of.

  In the present invention, after giving a temperature gradient in the longitudinal direction of the steel sheet with the first passage type cooling device, both the cooling start temperature and the cooling stop temperature in the second passage type cooling device are the maximum value in the longitudinal direction of the steel plate. The difference between the minimum values is preferably 50 ° C. or less.

  When the temperature difference exceeds 50 ° C., the difference in strength and toughness between the tip and tail ends becomes large. More preferably, it shall be 30 degrees C or less. In Case 3, quenching is performed by cooling b <b> 2 by the first passage type cooling device 10.

  The cooling method according to the present invention is applicable to any steel sheet having a composition suitable for quenching and tempering, but the component composition based on direct quenching described below is preferred. % In the component composition is mass%.

C: 0.01-0.20%
In order to secure the strength of the steel sheet, at least 0.01% is necessary for C, and if added over 0.20%, the weldability is remarkably lowered, so 0.01% or more and 0.20% or less (hereinafter, 0.01-0.20%).

Si: 0.01-0.80%
Si is an element necessary for deoxidation, but if less than 0.01%, the effect is small, and if added over 0.80%, the weldability and the base metal toughness are remarkably lowered, so 0.01-0. 80%.

Mn: 0.5-2.50%
Mn is necessary for securing the strength of the steel sheet in the same manner as C. If excessively added, the weldability is impaired, so 0.5-2.50% is set.

P: 0.020% or less, S: 0.0070% or less P and S are elements inevitably contained in the steel as impurities, in order to deteriorate the toughness of the steel base material and the weld heat affected zone. It is preferable to reduce as much as possible in consideration of economy, and P: 0.020% or less, S: 0.0070% or less.

Al: 0.004-0.10% or less Al is a deoxidizing element. If it is less than 0.004%, its effect is not sufficient, and if added excessively, toughness is deteriorated, so 0.004-0.10 % Or less.

  Although the preferable basic component composition of the present invention is as described above, when further improving desired characteristics, one or two of Ti, Cu, Ni, Cr, Mo, Nb, V, W, B, Ca, Mg, and REM are used. More than seeds are added as selective elements.

Ti: 0.005-0.20%
Ti has a predetermined range from the viewpoint of ensuring the toughness of the base metal and ensuring the toughness in the heat affected zone, but if added over 0.20%, the toughness is significantly reduced. 0.005 to 0.20%.

Cu: 0.01-2.0%
Cu is an element for increasing the strength and exerts its effect at 0.01% or more, and if added over 2.0%, the steel sheet surface properties deteriorate due to hot brittleness. 0.01-2.0%.

Ni: 0.01-4.0%
Ni can improve the toughness while increasing the strength of the base metal, and exhibits an effect at 0.01% or more, and the effect is saturated and economically disadvantageous at 4.0% or more. Is 0.01 to 4.0%.

Cr: 0.01-2.0%, Mo: 0.01-2.0%
Both Cr and Mo are effective in increasing the strength, and when 0.01% or more, the effect is exerted. When added over 2.0%, the toughness deteriorates remarkably. Each is 0.01-2.0%.

Nb: 0.003-0.1%, V: 0.003-0.5%
Nb and V are elements that improve the strength and toughness of the base material, and exhibit an effect when added in an amount of 0.003% or more. Further, if it exceeds 0.1% and 0.5%, respectively, the toughness may be lowered. Therefore, when added, Nb: 0.003-0.1%, V: 0.003-0.5 %.

W: 0.003-0.7%
W is an element that improves strength and corrosion resistance. If it is less than 0.003%, there is no effect, and if it exceeds 0.7%, the weld heat affected zone toughness may be deteriorated. Therefore, when adding, it is set as 0.003-0.7%.

B: 0.0005-0.0040%
B can increase the strength by improving the hardenability. This effect becomes prominent at 0.0005% or more, and the effect is saturated even if added over 0.0040%. Therefore, when added, the content is made 0.0005-0.0040%.

Ca: 0.0001-0.0060%, Mg: 0.0001-0.0060%, REM: 0.0001-0.0200%
Ca, Mg, and REM have a function of fixing S in steel and improving the toughness of the steel sheet, and are effective when added in an amount of 0.0001% or more. However, when adding over 0.0060%, 0.0060%, and 0.0200%, respectively, the amount of inclusions in the steel increases and deteriorates the toughness, so when added, Ca: 0.0001-0. 0060%, Mg: 0.0001-0.0060%, REM: 0.0001-0.0200%.

The balance other than the components described above consists of Fe and inevitable impurities. In the case of a steel plate, the molten steel having the above composition is melted by a conventional method using a melting means such as a converter or an electric furnace, and steel such as a slab by a conventional method such as a continuous casting method or an ingot-bundling method. It is preferable to use a raw material. The melting method and the casting method are not limited to the methods described above.
The thin-walled steel plate targeted by the present invention is suitable for construction machinery, has a plate thickness of 6 to 25 mm, a plate length of 20 to 50 m, a mechanical performance of 600 MPa or more, a steel plate tip and tail. The difference in tensile strength is ± 25 MPa and the difference in ductile-brittle fracture surface transition temperature (toughness) (vTrs) is within ± 10 ° C.

  If the plate thickness is less than 6 mm, the air cooling rate is high, and therefore, the rolling temperature, the cooling start temperature, and the stop temperature cannot all be made uniform. Further, when the plate thickness is 25 mm or more, the cooling rate at the time of air cooling is small, so that the temperature difference between the tip and tail ends of the steel plate is small without using the temperature gradient control method of the present invention. Regarding the plate length of the steel plate, if it is less than 20 m, the required product length may not be obtained, or the productivity is inferior, and even without using the temperature gradient control method of the present invention, The temperature difference at the tail end is small. Also, when the plate length of the steel plate exceeds 50 m, it takes time for rolling and cooling, the temperature drop at the tail end of the steel plate is large, the temperature difference between the tip and tail ends of the steel plate is large, and the temperature gradient control method according to the present invention This also makes it difficult to reduce the temperature difference between the tip and tail of the steel plate.

  If the difference in tensile strength between the tip and tail ends of the steel plate is outside ± 25 MPa, or if the difference in ductile-brittle fracture surface transition temperature is outside ± 10 ° C, the tensile strength and toughness of the entire steel plate will be within the target range ( The probability that it can be controlled within the range of (material standard) is lowered, the acceptance rate of the steel sheet is lowered, and the workability in the steel sheet becomes non-uniform. In addition, the production conditions such as the rolling finishing temperature and the cooling stop temperature are more constrained than necessary, which impedes productivity.

  A thick steel plate was manufactured by the manufacturing method according to the present invention, and the mechanical properties (strength and toughness) of the entire length of the steel plate were investigated. In the rolling-cooling facility shown in FIG. 1, the first passage type cooling device uses a cooling facility (FIG. 4C) in which the cooling area is 1 m between two draining rolls, and the second passage type cooling device is Direct quenching equipment was used.

  In this example, a steel plate having a thickness of 6 to 25 mm was produced from a 250 mm cross-section slab using various hot rolling conditions, and at a position in the vicinity of 500 mm from the foremost end and the end of the obtained steel plate, Tensile test pieces having a full thickness were collected and subjected to a tensile test in accordance with JIS Z 2241 (1998) to determine the tensile strength TS.

  Similarly, a Charpy impact test specimen having a V-notch standard dimension is taken from the position in the plate thickness direction 1/2 in accordance with the provisions of JIS Z 2202 (1998), and the JIS Z 2242 (1998) is designated. In accordance with the impact test, the ductile-brittle fracture surface transition temperature vTrs was determined. However, for a plate thickness of 11 mmt or less, vTrs was obtained using a half-size Charpy test piece.

  Table 1 shows the component composition of the test steel, and Tables 2 and 3 show the manufacturing conditions and the strength and toughness of the obtained steel sheet. In the inventive examples (Nos. 1 to 26), the TS difference between the tip and the tail became uniform within 25 MPa, and the toughness difference (vTrs difference) was also good within ± 10 ° C. On the other hand, in the comparative examples (Nos. 27 to 39), a uniform steel sheet with a large difference in TS and / or toughness between the tip and tail ends was not obtained.

DESCRIPTION OF SYMBOLS 1 Cooling tank 2 Reserved cooling water 3 Upper cooling water injection nozzle 4 Steel plate 5 Transport roll 6 Lower cooling water nozzle 7 Cooling area 8 Draining roll 9 Rolling mill 10 First passage type cooling device 11 Second passage type cooling device 12 Heating furnace 13 Rod cooling water

Claims (6)

A direct quenching type thin steel plate having the following steps using a rolling-cooling device in which a first passing type cooling device and a second passing type cooling device are arranged in order on the downstream side of a reversible hot rolling mill. Production method.
1. A step of applying a temperature gradient in the longitudinal direction of the steel plate with the first passage type cooling device before cooling the steel plate after finish rolling with the second passage type cooling device.
2. A step of quenching the steel sheet by passing a second passage type cooling device at a constant speed. When cooling with the first passage type cooling device, the temperature difference ΔT between the steel plate front end and the tail end after cooling is cooled so as to satisfy the expression (1) by changing the conveyance speed and / or the amount of water injection. The method for producing a direct-quenching thin-walled steel sheet according to claim 1.
15L / (hv) ≦ ΔT ≦ 55L / (hv) (1)
However, h: board thickness (mm), L: length (m), v: approach speed (m / s) to 2nd passage type cooling device When quenching with the second passage type cooling device in the step 2, the steel plate in the step 1 so that the cooling start temperature at the tip and tail ends of the steel plate is Ar ₃ points or more or a two-phase region temperature. The method for producing a direct-quenching thin-walled steel sheet according to claim 1 or 2, wherein the temperature at the front end in the longitudinal direction and the temperature at the tail end are adjusted.   When quenching with the second passage type cooling device in the step 2, the tip in the longitudinal direction of the steel plate in the step 1 so that the difference in cooling start temperature between the tip and the tail end of the steel plate is within 50 ° C. The method for producing a direct-quenching thin-walled steel sheet according to any one of claims 1 to 3, wherein the temperature of the part and the temperature of the tail end part are adjusted.   The composition of the steel sheet is mass%, C: 0.01 to 0.20%, Si: 0.01 to 0.80%, Mn: 0.50 to 2.50%, P: 0.020%. Hereinafter, S: 0.0070% or less, sol. The direct quenching thin-walled steel plate according to any one of claims 1 to 4, wherein Al: 0.004 to 0.100% is contained, and the balance is composed of Fe and inevitable impurities. Manufacturing method.   Further, as a component composition, steel: Ti: 0.005-0.20%, Cu: 0.01-2.0%, Ni: 0.01-4.0%, Cr: 0.01 -2.0%, Mo: 0.01-2.0%, Nb: 0.003-0.1%, V: 0.003-0.5%, W: 0.003-0.7%, One or more of B: 0.0005-0.0040%, Ca: 0.0001-0.0060%, Mg: 0.0001-0.0060%, REM: 0.0001-0.0200% The method for producing a direct-quenching thin-walled steel plate according to claim 5, comprising:

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