JP4959167B2 - Thermal processing method for steel sheet - Google Patents

Description

  The present invention relates to a method of heating and deforming a steel plate applied when manufacturing a streamlined welded structure by welding a steel plate having a plurality of curvature surfaces, particularly in the shipbuilding field, and in particular, the surface of the steel plate by a gas burner. Alternatively, the present invention relates to a linear thermal deformation method for a steel sheet that improves work efficiency when the back surface is linearly heated and then cooled by water to deform the steel sheet.

  Ship structures such as hulls in the shipbuilding field need to have a smooth surface with a continuous outer surface in order to reduce water flow resistance during voyage. A welded structure having a smooth and continuous curvature surface is formed by welding each other.

  Since this steel plate needs to be bent with a complex and delicate curvature depending on the part of the ship structure, it cannot be handled by simple and uniform press work. Has been done. This linear heating deformation is a method in which a steel plate is locally heated linearly using a gas burner or the like, and water cooling is performed immediately after heating.

  In the actual manufacturing of a ship structure, in order to obtain a smooth curvature surface, a plurality of locations on the steel plate surface are linearly heated and deformed, and the same location is repeatedly subjected to linear heating and water cooling a plurality of times. Moreover, since these are all performed by the intuition and skill of a skilled person, it may take 5 days or more to bend a single steel plate into a predetermined shape. Among them, the hull head part is the part that directly receives resistance from seawater, and it is necessary to process it into a curved surface with a smaller radius of curvature to reduce the resistance, and it takes a long time for the processing process, so labor costs and This is a major factor in increasing shipbuilding costs. For this reason, from the viewpoint of reducing shipbuilding costs and shortening the number of manufacturing days, improvement and automation of bending work efficiency by linear heating deformation of steel sheets have been studied conventionally.

  In recent years, with the aim of improving the work efficiency of bending work by linear heat deformation of steel plates, and further aiming for automation, predicting the heat deformation at the time of linear heat deformation of steel plates using numerical analysis by the finite element method (FEM), A method for determining a heating condition for deformation into a target shape has been proposed (see, for example, Patent Document 1 and Patent Document 2). However, since the accuracy of the estimation formula by numerical analysis of the finite element method (FEM) is not sufficient, the target shape may not be obtained under the heating conditions set based on this, and each time according to changes in the work environment and heating conditions. The estimation formula must be corrected, and automation has not yet been realized.

  On the other hand, from the viewpoint of the linear heating characteristics of a steel plate, techniques relating to a steel plate that can be easily bent during linear heating have also been proposed (Patent Documents 3 and 4). However, in order to dramatically improve the working efficiency when bending a steel sheet by linear heating, it is necessary to sufficiently improve the lateral deformation characteristics of the linear heating characteristics. Although effective in improving the deformation characteristics, it has been difficult to sufficiently improve the lateral deformation characteristics of the steel sheet. For this reason, in the conventional steel plate, it is the present condition that it has not led to improving the working efficiency of the bending process by linear heating dramatically. From such a state of the art, there is a demand for a method for stably controlling the heating deformation of the bending process by the linear heating deformation of the steel sheet and further improving the working efficiency.

JP 09-285823 A JP 09-323129 A JP 07-136720 A Japanese Patent Laid-Open No. 07-138715

  In view of the problems of these prior arts, the present invention is mainly applied in the case of manufacturing a marine welded structure having a complicated and delicate curvature surface, and has a large amount of lateral deformation due to lateral contraction even under the same linear heating condition. It is an object of the present invention to provide a heating deformation method that can bend a steel sheet into a target shape with higher work efficiency than before, by selecting a steel sheet having a thickness and limiting linear heating deformation conditions.

  The present invention solves the technical problems of the above prior art, and the gist of the invention is as follows.

(1) the front or rear surface of the steel sheet by a gas burner heating a linear, continued thermal processing of steel sheet up to the radius of curvature machined so as not to exceed 4.0m to the heating unit by causing bending deformation of the steel sheet by water cooling In the method, as the steel sheet, by mass%, C: 0.001 to 0.048 %, Si: 0.005 to 0.4%, Mn: 0.05 to 0.6%, Al: 0.002 0.12%, N: 0.001 to 0.01%, P: 0.03% or less, S: 0.01% or less, the balance is iron and inevitable impurities, yield at room temperature strength is 200～300MPa, and using a steel tensile strength of 250~400MPa at room temperature, in a vertical direction with respect to one of the sides of the steel plate, and the steel sheet except for the central portion surface or back on Range, the highest of the heating part By performing linear heating so that the temperature is 900 ° C. or lower, a plurality of linear heating regions are formed at intervals along the side, and shrinkage deformation of the linear heating region occurs in a direction along the side. A method of thermally processing a steel sheet, comprising bending and deforming the steel sheet.

  (2) The steel sheet thermal processing method according to (1) above, wherein the steel sheet contains 20 to 95% of a ferrite phase in which dislocations are introduced by processing or transformation strain in the microstructure.

  (3) The said steel plate is further mass%, Ti: 0.0005-0.02%, V: 0.005-0.1%, Nb: 0.0005-0.05%, and Mo: 1 type or 2 types or more of 0.05-0.3% are contained, The heat processing method of the steel plate of the said (1) or (2) description characterized by the above-mentioned.

  (4) The above steel sheet further comprising 0.05 to 0.3% in total of one or more of Ni, Cu and Cr in mass%. (1) -The heat processing method of the steel plate in any one of (3).

  (5) The above steel sheet further containing 0.0005 to 0.03% of the total amount of Mg, Ca, and REM in one or more of Mg, Ca, and REM in mass%. (1) -The thermal processing method of the steel plate in any one of (4).

  According to the present invention, a work process in heating deformation, which is mainly applied when manufacturing a marine welded structure having a complicated and subtle curvature surface, has been required to repeatedly perform many heating and cooling work processes so far. The working efficiency can be dramatically improved as compared with the prior art, and the industrial contribution such as the shipbuilding field according to the present invention is very large.

  The present invention is described in detail below.

  In general, bending by linear heating deformation of a steel plate mainly used in the manufacture of a marine welded structure is performed as follows.

  That is, when a heating source such as a gas burner is usually used to locally heat a predetermined region on the front or back surface of the steel sheet in a linear manner, and the heating region shrinks by cooling after thermal expansion, the surrounding non-heating region The steel plate is plastically deformed by the restraint from. The steel sheet can be processed by introducing the plastic deformation into the steel sheet in accordance with the intended processing shape. However, when the intended final shape is complicated, it is often not always clear how each part heated by the heat deformation characteristics of the steel plate is deformed, and it is difficult to predict.

  In general, in bending deformation by linear heating of a steel sheet, two types of deformation modes, angular deformation and lateral deformation, are largely used due to the difference in heat shrinkage of the steel sheet.

  FIG. 1 is a conceptual diagram for explaining two types of deformation modes of lateral deformation (A) and angular deformation (B) by linear heating of a steel plate.

  As shown in FIG. 1 (A), the horizontal deformation of the steel sheet by linear heating is performed when the difference between the amount of lateral shrinkage 10 is small in the thickness direction of the steel sheet 9 as shown in FIG. It is a deformation that shrinks in the horizontal direction as shown in FIG.

  Further, as shown in FIG. 1 (B), the angular deformation due to linear heating of the steel sheet is relatively larger than the other as shown in FIG. In this case, as shown in (d), it is a deformation that bends to the steel plate surface side with a large amount of lateral shrinkage.

  The lengths of the arrows in FIGS. 1 (A) and 1 (B) indicate the magnitude of the amount of lateral shrinkage 10. In both cases of lateral deformation and angular deformation, the surface of the steel sheet heated linearly However, due to the difference in the amount of local shrinkage in the thickness direction of the steel sheet, the deformation of the entire steel sheet is greatly different.

  Conventional steel plates for bending by linear heating (see, for example, Patent Documents 3 and 4) increase the strength of steel plates particularly at medium to high temperatures of about 500 to 600 ° C., and during linear heating, FIG. As shown, the difference in lateral shrinkage generated between the heated steel plate surface and its back surface is increased, and the bending amount in the angular deformation mode is improved. That is, the conventional steel sheet improves the high-temperature yield strength of 500 to 600 ° C., reduces the amount of lateral shrinkage on the back surface while ensuring the amount of lateral shrinkage on the heating surface shown in FIG. And the difference in lateral shrinkage between the back surface and the back surface thereof are relatively increased to promote angular deformation. However, a conventional steel plate does not necessarily have a sufficient effect in order to improve the work efficiency of bending the steel plate by linear heating. The reason will be described below.

Thermal processing of the steel plate using the angular deformation mode shown in FIG. 1 (B), for example, as shown in FIG. 2 (A), a linear heating region vertical direction with respect to the sides of the steel sheet, It is formed along the side at a predetermined interval, causes an angular deformation due to a difference in lateral shrinkage between the front and back surfaces of the heating unit, and is bent and deformed in an arc shape when viewed from the direction 1. Conventional steel plates with increased yield strength at medium and high temperatures (see, for example, Patent Documents 3 and 4) can be expected to improve certain working efficiency in the thermal processing of steel plates using such an angular deformation mode.

  However, the thermal processing of a steel sheet using such an angular deformation mode is not necessarily sufficient for improving the efficiency when bending the steel sheet into the intended shape for the following reasons. I can not say. That is, in the thermal processing of the steel sheet using the angular deformation mode, for example, when viewed from the direction 1 shown in FIG. 2 (B), the bending shape on the arc having a curvature radius 11 shown in FIG. Although it is obtained, the bent shape of the steel sheet cannot be obtained when viewed from the direction 2 shown in FIG. For this reason, for example, in order to obtain a bent shape with a radius of curvature equal to or less than a predetermined value when viewed from both the direction 1 and the direction 2 such as the hull head portion, the direction 2 is also bent by linear heating as described above. It is necessary to apply processing. However, since the bending rigidity of the steel sheet is increased after the heat processing into the shape shown in FIG. 2 (B) using the angular deformation due to the linear heating, the steel sheet by the linear heating is increased. The bending efficiency becomes difficult, and the working efficiency decreases, for example, the number of repeated heating increases.

  When manufacturing ship structures such as hulls in the shipbuilding field, it is necessary to make the outer surface of the steel plate have a smooth and smooth curvature surface in order to reduce water flow resistance during voyage. It is necessary to bend the steel plate into a bent shape with a smaller radius of curvature in order to reduce the resistance of the hull head portion directly received. In order to heat-process a steel plate into such a complicated bent shape with a smaller curvature radius, the conventional steel plate has a limit to sufficiently improve the working efficiency.

  In the present invention, when manufacturing a conventional ship welded structure, improvement in work efficiency is particularly desired, and when performing bending work that requires a small radius of curvature such as the front part of the hull, the work efficiency of heating work is reduced. The method of improving was studied earnestly.

  As a result, it is assumed that the work efficiency should be improved in the past, and the bending of the final shape with a maximum curvature radius of 4.0 m or less is assumed. It became clear that it is necessary to presume deformation processing using lateral deformation, not bending deformation processing using deformation. In the present invention, in order to improve the working efficiency of the bending process with the maximum curvature radius of the final shape of 4.0 m or less, a steel plate having a large amount of lateral deformation at the time of linear heating is used, and the lateral deformation at the time of linear heating is performed. By promoting, the technical idea is to reduce the number of heating repetitions when bending deformation to the final shape, and to improve work efficiency.

  First, a method of performing bending with a maximum curvature radius of 4.0 m or less using a transverse deformation mode of a steel sheet during linear heating assumed in the present invention will be described.

  As described above, the transverse deformation mode by linear heating of the steel sheet is as follows. When the difference in lateral shrinkage is small in the sheet thickness direction of the steel sheet as shown in FIG. Deformation that shrinks in any direction. Since the lateral deformation of the steel sheet by linear heating is in-plane deformation as shown in FIG. 1 (A), just by linearly heating one place of the steel sheet, the angular deformation mode shown in FIG. Cannot cause out-of-plane deformation. However, it is possible to bend and deform the steel sheet into a shape with the desired curvature by adjusting the conditions such as the position of the linear heating region and the non-heating region, the interval and width of the linear heating region as follows: It becomes.

Of the steel plate using the transverse deformation mode bending, for example, as shown in FIG. 3, with respect to the sides 1 and side 2 of the rectangular steel plates in vertical direction, and the steel sheet surface or on the back surface except the central portion Are heated linearly, and a plurality of linear heating regions 3 (on both sides of side 1 and side 2 excluding the central portion (non-heated region) in the linear heating direction are spaced along sides 1 and 2. The hatched portion in the figure is formed. In the respective linear heating regions 3 formed on both sides of the side 1 and the side 2 of the steel plate, lateral shrinkage occurs due to cooling after heating, and lateral shrinkage 5 occurs on the sides 1 and 2 of the steel plate due to thermal processing, resulting in lateral deformation. Occurs. As a result, the lateral direction (perpendicular to the linear heating direction) on both sides (linear heating region 3) of side 1 and side 2 of the steel plate with respect to the central portion (non-heated region) of the steel plate not subjected to linear heating. The direction is relatively short, and the steel plate is bent (out-of-plane).

  As a result of the study by the inventors, when bending a steel sheet having a small maximum curvature radius of the final shape by linear heating, it is better to use the lateral deformation mode than to use the angular deformation mode. It was confirmed that it is effective for improving the work efficiency of bending. That is, as described above, in the bending process of the steel sheet using the angular deformation mode, the bending radius is not more than a predetermined value when viewed from either the direction 1 or the direction 2 shown in FIG. In order to obtain the shape, it is necessary to first perform the bending process shown in FIG. 2 (A) by linear heating and then perform the same bending process by linear heating as viewed from direction 2. Arise. However, after processing into the bending shape shown in FIG. 2 (B), the bending rigidity of the steel sheet increases compared to before processing, so bending of the steel sheet by linear heating becomes difficult and the number of repetitions of heating increases. As a result, work efficiency decreases.

  On the other hand, in the bending process of the steel sheet using the lateral deformation mode, conditions such as the positional relationship between the linear heating region 3 and the non-heating region, the interval between the linear heating regions 3 as shown in FIG. 3 are adjusted. Thus, as shown in FIG. 3B, it is possible to bend into a bent shape having a predetermined radius of curvature 11 when viewed from two directions (arrow 4). For this reason, in bending of steel plates using the transverse deformation mode, bending rigidity after processing, such as bending of steel plates using the angular deformation mode, even when bending with a small radius of curvature such as the top part of the hull, is performed. As a result, there is no problem of reduction in work efficiency such as increase in the number of heating repetitions due to the increase in work efficiency, and work efficiency can be improved.

Further, it has been found that the improvement in work efficiency by bending the steel sheet using the transverse deformation mode has a particularly advantageous effect under the condition that the maximum temperature reached by the heating part during linear heating is 900 ° C. or less.
The present invention has been made on the basis of the above knowledge, and the component composition, room temperature yield strength, and room temperature tensile strength were defined when bending so that the maximum curvature radius of the final shape was 4.0 m or less. By using a steel plate, the entire temperature of the heating unit is uniformly shrunk in the direction of the steel plate surface by linear heating with a maximum temperature of the heating unit of 900 ° C. or less (in the thickness direction shown in FIG. 1 (A)). The technical idea is to improve the working efficiency of bending by increasing the amount of lateral deformation.

  Below, based on the above technical idea, in the thermal processing method of the steel sheet of the present invention, the component composition of the steel sheet having a high amount of lateral deformation at the time of linear heating, the mechanical properties, and the conditions for the linear heat deformation, more preferably The limitation of the structure of the steel sheet structure will be described.

(Maximum curvature radius of final shape)
The reason why the upper limit of the maximum curvature radius of the final shape is set to 4 m in the present invention will be described below.

  An object of the present invention is to use a steel plate having a large amount of lateral deformation during linear heating, and to improve the working efficiency of the steel plate bending process. The effect of improving the working efficiency is reduced by the radius of curvature of the final shape. It is not a thing. However, as the radius of curvature of the final shape increases, a significant difference in effect from a bending method using a steel sheet having a large amount of angular deformation during conventional linear heating (for example, see Patent Documents 3 and 4) decreases. The maximum curvature radius of the final shape by thermal processing, which can obtain the advantageous effect of the work efficiency of the steel sheet bending process of the present invention, was defined as 4 m or less.

  As described above, when bending the steel sheet by utilizing the angular deformation caused by linear heating, when the maximum curvature radius of the final shape is as small as 4 m or less, it is viewed from the direction 1 shown in FIG. The rigidity increases when processing into the bent shape shown in FIG. 2A, and the workability when the same bending process is performed next as seen from direction 2 is reduced. As a result, the work efficiency decreases such as an increase in the number of heating repetitions. Arise.

  In the present invention, even when the maximum radius of curvature of the final shape is as small as 4 m or less, a steel plate having a large amount of lateral deformation due to linear heating is used, and two directions (direction 1 and direction) shown in FIG. Since the bending process shown in FIG. 2A as viewed from 2) can be achieved at the same time, the working efficiency can be greatly improved as compared with the conventional method.

(Yield strength (YP) and tensile strength (TS) of steel plate)
Next, the reasons for limiting the yield strength and tensile strength of the steel sheet in the present invention will be described.

  The thermal deformation of the steel sheet due to linear heating is a phenomenon in which when the heating part shrinks by cooling after thermal expansion, the heating part of the steel sheet yields due to restraint from the surrounding non-heated area, and plastic deformation occurs. This is related to the yield strength of the steel sheet. In the present invention, it is a technology to improve the work efficiency of bending work with a small maximum radius of the final shape by using a steel plate having a large amount of lateral deformation during linear heating without changing the linear heating condition and cooling condition. It is thought. As shown in FIG. 1 (A), the thermal characteristics of the steel sheet, in which the amount of lateral deformation increases during linear heating, the difference in local lateral shrinkage between the front and back surfaces of the heating unit is small and uniform in the thickness direction. In particular, those that increase the amount of lateral shrinkage are preferred. For this reason, the steel sheet used in the present invention is more likely to yield even on a non-heated surface at a low temperature, compared to a directly heated surface where the maximum temperature reached 900 ° C. or less during linear heating, and sufficiently introduces plastic strain. It is necessary to increase the amount of lateral shrinkage uniformly in the thickness direction of the part. In order to obtain this effect, in the present invention, a steel plate having an upper limit of yield strength (YP) of 300 MPa needs to be used as a steel plate that easily yields even at a low heating temperature of the non-heated surface. When the yield strength (YP) at room temperature of the steel sheet is higher than 300 MPa, it is difficult to yield on the non-heated surface, which is at a lower temperature than the directly heated surface, and the amount of lateral shrinkage is small. Angular deformation is likely to occur on the heating surface side, and the effect of increasing the amount of lateral shrinkage uniformly in the thickness direction of the heating portion and improving the work efficiency of bending deformation using lateral deformation cannot be obtained sufficiently.

  On the other hand, the lower limit of the yield strength of the steel sheet is not particularly limited from the viewpoint of the work efficiency of bending deformation using lateral deformation, but when the yield strength of the steel sheet is less than 200 MPa, the steel sheet component is made of pure iron. It is not preferable because the components must be close to each other, which increases the manufacturing cost of the steel sheet.

  For the above reasons, in the present invention, the yield strength of the steel sheet is limited to 200 MPa to 300 MPa.

  In the present invention, as described above, since the bending is performed under the condition that the maximum curvature radius of the final shape is as small as 4 m or less, the steel sheet is bent and deformed to the maximum curvature radius of the final shape as shown in FIG. It is necessary to carry out a thermal processing treatment for repeatedly heating and cooling the same portion of the linear heating region 3 many times. With the increase in the number of repetitions of the linear heating and cooling thermal processing, work hardening occurs due to an increase in the amount of plastic strain introduced into the heating region 3, that is, the thermal processing section. As a result, the yield strength of the steel sheet in the heat-processed portion increases as the number of heat-processing repeats increases, so the amount of lateral deformation of the heated portion due to linear heating decreases, and the work efficiency of bending decreases. End up.

  Therefore, in the present invention, in order to suppress the work hardening caused by the amount of plastic strain introduced by the linear heating and cooling thermal processing, in addition to the above-mentioned definition of the yield strength of the steel plate, the tensile strength of the steel plate is set. It is necessary to specify as follows.

  FIG. 4 shows two types of steel plates having the same yield strength (280 MPa) and different tensile strengths (◯: 340 MPa, ●: 440 MPa) when linear heating and cooling are repeatedly performed by thermal processing. The relationship between the number of repetitions and the yield strength of a heat-processed part is shown. In addition, the thermal processing of the steel sheet is performed by linear heating using a gas burner under the condition that the maximum heating temperature of the heating part of the steel sheet is 600 ° C., then water cooling, and this linear heating and cooling are repeated a predetermined number of times. The yield strength of the heat-processed part was measured.

  As shown in FIG. 4, the yield strength of each steel sheet increases with the work hardening of the heat-processed part as the number of repetitions of thermal processing of linear heating / cooling increases, but the maximum of each yield strength is It converges to the tensile strength of the steel sheet and does not exceed the tensile strength. For example, at first, the yield strength of all the steel plates is 280 MPa, but when the number of thermal processing is 10 times, the yield strength of the heat-processed portion is up to 311 MPa in the case of a steel plate (◯) with a tensile strength of 340 MPa. In the case of a steel plate (●) having a tensile strength of 440 MPa, it increases to 357 MPa. In other words, even when using a steel plate having a yield strength that falls within the same scope of the present invention, the yield strength increases due to work hardening of the heat-processed portion with an increase in the number of repetitions of the heat-processing treatment. The degree depends on the initial yield strength of the steel sheet. The steel plate with high tensile strength has a higher yield strength increase due to thermal processing per time than the steel plate with low tensile strength. Therefore, especially in the bending work under the condition that the maximum curvature radius of the final shape is small. The decrease in efficiency is significant.

  In the present invention, based on the above examination results, in order to sufficiently improve the working efficiency of bending work under the condition that the maximum power factor radius of the final shape is as small as 4 m or less, the yield strength at room temperature of the steel sheet is specified. In addition, by defining the upper limit of the tensile strength of the steel sheet to 400 MPa, it is possible to reduce the work efficiency while reducing the work efficiency due to the increase in the number of repetitions of the thermal processing, the increase in the yield strength, and the final heat It is necessary to increase the lateral deformation amount of the processed part sufficiently. Even if the yield strength at room temperature of the steel sheet is within the range of the present invention, when the upper limit of the tensile strength of the steel sheet exceeds 400 MPa, it results from an increase in the yield strength performed to increase the number of repetitions of the thermal processing. The reduction in work efficiency becomes significant, and the work efficiency of bending up to the maximum power factor radius of the final shape cannot be sufficiently improved.

  On the other hand, the lower limit of the tensile strength of the steel plate is not particularly limited from the viewpoint of the work efficiency of bending deformation using lateral deformation, but when the tensile strength of the steel plate is less than 250 MPa, the steel plate component is changed to pure iron. It is not preferable because the components must be close to each other, which increases the manufacturing cost of the steel sheet.

  For the above reasons, in the present invention, the tensile strength of the steel sheet is limited to 250 MPa to 400 MPa.

(Linear heating and cooling conditions, maximum temperature reached)
Next, the reason for limitation of the present invention will be described for heating and cooling.

  In the present invention, after the steel sheet is linearly heated using a gas burner, the bending process with the maximum radius of curvature of the final shape being not more than a predetermined value is performed by repeating the water-cooling thermal processing. Water cooling after linear heating not only shortens the cooling time, but also suppresses the decrease in yield stress by suppressing the temperature rise of the surrounding non-heated part when the linear heating part contracts laterally. It is necessary to apply a strong restraining force. In the present invention, since the strength of the steel material is limited to be low, water cooling is indispensable in order to maintain the restraining force from the periphery of the heating unit as high as possible. Cooling is also necessary for grasping the amount of bending after the thermal processing and facilitating correction of the next thermal processing conditions.

  Generally, in a bending process by linear heating of a steel sheet, the amount of plastic deformation during one thermal processing tends to increase as the heat input of the linear heating part increases. This is because the area of expansion and contraction per time becomes wider as the heat input amount of the linear heating unit increases. However, the increase in heat input, that is, the maximum ultimate temperature of the steel sheet during linear heating increases and the heating time becomes longer, so the work efficiency when bending to the final shape under excessively high ultimate temperature conditions Will drop.

  In the present invention, as described above, by defining the yield strength and tensile strength of the steel material, while improving the amount of lateral deformation in the thermal processing per time, the yield strength accompanying the increase in the number of repetitions of the thermal processing. Since the increase can be suppressed, the working efficiency of the bending process can be sufficiently improved even under the condition where the linear heating temperature is lower than the conventional one. However, when the maximum temperature of the steel sheet exceeds 900 ° C. during linear heating, the amount of deformation obtained by one heat processing can be increased, but the heating time becomes longer, resulting in repeated heat processing. Thus, it is difficult to sufficiently improve the working efficiency when bending the final shape. For example, even if the amount of deformation obtained in one thermal processing operation is doubled, if the working time is tripled, the efficiency is lowered as a whole, which is not preferable. Therefore, in the present invention, the maximum temperature reached by the steel sheet heating part during linear heating is limited to 900 ° C. or less.

  Next, the reasons for limiting the component composition of the steel sheet in the present invention will be described.

  Unless otherwise specified, “%” shown below means “% by mass”.

C is an element that increases the strength of the steel sheet and increases the hardenability. If the lower limit of C, 0.001%, is less than C, the steel sheet strength cannot be set within the range of the present invention. In the present invention, since the radius of curvature is limited, it is limited to a portion where a load is not applied so much, but since a minimum strength is necessary, this value is set as the lower limit. In general, the strength of a steel sheet depends not only on C but also Si, Mn, etc., but when C exceeds 0.06%, the strength increases to the same level as that of a normal steel sheet, so the upper limit is set to this value. However, the maximum C amount of the present invention example shown in the examples is 0.048% .

  Si is a deoxidizing element and also a strength maintaining element. If the upper limit of 0.4% of Si exceeds this value, the strength is improved due to an increase in the amount added, so this value was set. The lower limit of 0.005% was set as the lowest level at which the deoxidation effect can be exhibited.

  Mn is an element that ensures the same strength as C. 0.05% of the lower limit was set as the lowest value from the viewpoint of securing strength. In addition, the upper limit of 0.6% is set at this value because the added amount exceeding the upper limit results in the strength exceeding the strength range of the present invention.

  P and S are impurities in the present invention. However, if these elements are contained excessively, the toughness of the steel sheet and HAZ deteriorates, so the upper limits were made 0.03% and 0.01%, respectively.

  Al is a deoxidizing element like Si. For this purpose, 0.002% was set as the lower limit. When lower than this, the oxygen in a steel plate will increase and the problem on a steel plate characteristic will arise. Moreover, since excessive addition impairs weldability, the upper limit is made 0.12%.

  In a small amount, N forms fine nitrides when the steel slab is heated to refine the heated austenite grain size and contribute to toughness. For that purpose, 0.001% or more is necessary as content in steel. On the other hand, if the content exceeds 0.01%, the nitride becomes coarse, or the solid solution N increases and the toughness is deteriorated, so the upper limit was set to 0.01%.

  The above is the basic component of the steel sheet in the present invention, but in order to adjust the strength of the steel sheet, one or more of the following Ti, V, Nb and Mo may be added in the following content. it can.

  Ti is a precipitation strengthening element. In the case where the steel sheet strength is adjusted within the range of the present invention, if it is effectively used, the amount of Mn added can be reduced accordingly. However, if the addition amount exceeds 0.02%, the strength increases remarkably, and the strength cannot be kept within the range of the present invention, so the upper limit was made 0.02%. For the lower limit of 0.0005%, the effect of ensuring the strength of the steel sheet is lost at an addition amount lower than this, so this value was set as the lower limit.

  V, like Ti, is a precipitation strengthening element. However, the effect is small compared to Ti. Therefore, even if it adds more than the upper limit of Ti, steel plate strength can be set within the range of the present invention. However, when the added amount exceeds 0.1%, the strength increase is too large, and the steel plate strength cannot be kept within the range of the present invention, so this value was set. Since 0.005% of the lower limit loses the effect of securing the strength of the steel sheet when the addition amount is less than this, this value is set as the lower limit.

  Nb is also an element having the same function as Ti and V. However, since the magnitude of the effect is different from Ti and V, the range needs to be set in accordance with Nb. When the added amount exceeds 0.05%, the strength increase is too large, and the steel plate strength cannot be kept within the range of the present invention, so this value is set. For the lower limit of 0.0005%, the effect of ensuring the strength of the steel sheet is lost at an addition amount lower than this, so this value was set as the lower limit.

  Mo is also an element having the same function as Ti, V, and Nb. However, since the magnitude of the effect is different from those of these elements, it is necessary to set the range according to Mo. The lower limit of Mo, 0.05%, is less than this, so this effect is set because the effect of improving the strength cannot be obtained. This value was set at 0.3% of the winged base because the steel sheet strength at room temperature exceeded the range of the present invention at an addition amount exceeding this, and the thermal processing efficiency deteriorated.

  In the present invention, one or more of Ni, Cu and Cr may be added in a total amount of 0.05% to 0.3%.

  Unlike Ti, V, Nb, and Mo, these elements contribute to improving the strength of the steel sheet by increasing the hardenability of the steel sheet. Generally, Cr is also a precipitation strengthening element, but this is a case where the addition amount is 0.5% or more, and precipitation hardening cannot be expected so much in a region where the addition amount is small as in the present invention. In the addition amount of the present invention, the contribution of Cr is a contribution by improving hardenability. In this case, since the functions of Ni, Cu, and Cr are almost the same, the addition amount of these elements is defined by the total amount. The lower limit of 0.05% was set at this value because the addition amount below this value had no effect on the steel sheet strength. The upper limit of 0.3% is set at an addition amount exceeding this value, because the room temperature strength becomes too high, the thermal processing efficiency deteriorates, and is indistinguishable from the conventional steel plate.

  In this invention, in addition to the said component, 1 type (s) or 2 or more types among Mg, Ca, and REM can be added as needed. The purpose of these element additions is to improve the ductility of the steel sheet and to improve the HAZ toughness after welding. In order to ensure the effect, it is necessary to add 0.0005% or more of these elements in total. On the other hand, excessive addition causes coarse inclusion formation, and the upper limit is set to 0.03% in total.

(Structure of steel plate with high lateral deformation)
Next, the reason why the microstructure of the steel sheet of the present invention is limited will be described.

  In the present invention, as described above, by using a steel plate with a limited yield strength, tensile strength, and component composition, work when performing bending by thermal processing under a condition where the radius of curvature of the final shape is as small as 4.0 m or less. The efficiency can be greatly improved compared to the conventional case.

  The steel sheet is not particularly limited in order to achieve the intended effect of the present invention, but in order to obtain a more reliable effect, it is preferable to limit the microstructure of the steel sheet to the following structure.

  The amount of plastic deformation of the steel plate in the linear heating of the steel plate by the gas burner and the thermal processing by water cooling is substantially controlled by the yield strength and tensile strength of the steel plate. However, even for steel sheets with the same yield strength and tensile strength at room temperature, the high temperature strength during linear heating, that is, the temperature dependence of the steel sheet strength, depends on the microstructure of the steel sheet, so it can be bent to the final shape by repeated thermal processing. In order to improve the working efficiency when processing, it is desirable to limit the microstructure of the steel sheet.

  In the present invention, as a thermal characteristic of a steel sheet that has a large amount of lateral deformation during linear heating, it is more likely to yield even on a non-heated surface that is at a lower temperature than a directly heated surface where the maximum temperature reached 900 ° C. or less during linear heating. A steel plate in which plastic strain is sufficiently introduced, and as shown in FIG. 1A, the difference in local lateral shrinkage between the front and back surfaces of the heating part is small, and the lateral shrinkage increases uniformly in the thickness direction. Use. For this reason, it is preferable that the steel plate used in the present invention has a microstructure that easily yields even on a non-heated surface at a low temperature. The steel sheet having such thermal characteristics basically does not use or suppress the amount of elements that increase the high-temperature strength such as Mo and Nb, and secures the strength at room temperature by the structure control, at about 400 to 600 ° C. A microstructure in which the yield stress is sufficiently low with respect to the yield stress at room temperature is preferred. Fine grain strengthening, solid solution strengthening, precipitation strengthening, dislocation strengthening, etc. are known to ensure the strength of steel sheets at room temperature, but compared to the contribution to yield stress at room temperature, during heating during thermal processing Dislocation strengthening is the most preferable as a strengthening factor that reduces the contribution of yield stress.

  For these reasons, the structure of the steel sheet of the present invention preferably contains 20 to 95% of a ferrite phase in which dislocations are introduced by processing or transformation strain in the microstructure.

  A steel sheet strengthened by introducing dislocations into such a ferrite matrix increases the yield stress at room temperature due to dislocation strengthening, but the dislocations disappear at a relatively high temperature during heating during thermal processing. Since the rearrangement significantly reduces the contribution to strengthening, the ratio of the yield stress during heating to the yield stress at room temperature can be effectively reduced.

  The ferrite phase, which is contained in the microstructure of the steel sheet and into which dislocations are introduced by processing or transformation strain, secures the yield stress at room temperature of the steel sheet and at the same time lowers the yield stress during heating of the steel sheet. If the ratio of ferrite into which dislocations are introduced by processing or transformation strain in the steel sheet is less than 20%, the yield stress at the time of heating relative to the yield stress at room temperature of the steel sheet is not sufficient, and if it exceeds 95%, The yield stress at room temperature is too high.

  Therefore, in the present invention, the content of the ferrite phase in which dislocations are introduced into the microstructure of the steel sheet by processing or transformation strain is limited to 20 to 95%.

  In the present invention, the second phase other than the ferrite phase in the steel sheet structure is not particularly limited, and examples thereof include precipitates such as pearlite, bainite, martensite, and carbonitride.

  An example of an embodiment of a method for producing a steel sheet satisfying the yield strength, tensile strength and component composition defined in the present invention will be described below.

  The introduction of the working dislocations into the ferrite matrix of the steel sheet of the present invention is as follows. After heating the steel piece having the component composition defined in the present invention and rolling it in the austenite region, (1) a two-phase region having a predetermined ferrite fraction (Ferrite / austenite coexistence region)-Ferrite temperature range, rolling at a predetermined cumulative reduction rate, or (2) Rapid cooling from a temperature range where a predetermined ferrite fraction is reached to a predetermined temperature, and thermal expansion and contraction Can be achieved.

Specifically, the steel sheet of the present invention can be produced by the following hot rolling method.
(A) The steel slab is heated to a temperature not lower than the A _C3 transformation point and not higher than 1300 ° C., and after performing austenitic rolling with a cumulative reduction of 50% or higher in the temperature range not lower than the Ar3 transformation point, the ferrite fraction is 20 % Phase rolling with a cumulative rolling reduction of 10 to 75% in a temperature range of at least%.
(B) The rolling reduction of each rolling pass in the two-phase region rolling is set to 15% or less.
(C) After completion of the rolling, accelerated cooling is performed at a cooling rate of 3 to 100 ° C./s up to a temperature range of 400 ° C. or lower.

Alternatively, when the hot rolling is terminated in the austenite region without performing the two-phase region rolling,
(D) The steel slab is heated to a temperature not lower than the A _C3 transformation point and not higher than 1300 ° C., and subjected to austenite rolling with a cumulative reduction of 50% or higher in the temperature range not lower than the Ar3 transformation point. Is accelerated and cooled at a cooling rate of 3 to 100 ° C./s from a temperature range of 50% or higher, from a temperature range of 500 ° C. or higher to a temperature range of 400 ° C. or lower.
The conditions of the manufacturing methods (a) to (d) above will be described in detail below.

In the above (a), the reason why the heating temperature of the steel slab is set to a temperature not lower than the AC ₃ transformation point and not higher than 1300 ° C. is to uniformly austenite the steel material structure. If the heating temperature is less than the A _C3 transformation point, it does not become 100% austenite, so that the final structure of the steel sheet is not uniform, and the variation of the steel sheet material becomes significant, which is not preferable. On the other hand, when the heating temperature is higher than 1300 ° C., the heated austenite grain size becomes excessive, and thereafter, it becomes difficult to sufficiently refine the structure even by hot rolling, and the toughness of the steel sheet may be deteriorated. For this reason, in this invention, the heating temperature of a steel slab shall be _AC3 transformation point or more and 1300 degrees C or less.

  Even if the cast steel slab in a high temperature state is directly rolled as it is, or even if the cast steel slab is heated in a heating furnace while being cooled to room temperature, the steel slab cooled to the above room temperature is heated. The above effect is the same as the case.

In addition, the austenite region rolling in the above (a) is performed in order to uniformly refine the austenite grain size before ferrite transformation and to uniformly refine the ferrite transformation structure in order to ensure the material of the steel sheet, particularly toughness.
The austenite region rolling condition needs to be performed in a temperature range equal to or higher than the _Ar3 transformation point and a cumulative reduction ratio of 50% or more. The reason why the rolling is performed in the temperature range _{equal to} or higher than the _Ar3 transformation point is that the rolling is completed in the austenite region as distinguished from the effect of the subsequent two-phase region rolling. As long as the austenite zone rolling is controlled with respect to the temperature and the cumulative reduction ratio of the subsequent two-phase zone rolling, there is no need to provide a time interval with the subsequent two-phase zone rolling. Further, the reason why the cumulative rolling reduction in the above temperature range is 50% or more is that if this cumulative rolling reduction is less than 50%, depending on the rolling temperature zone, the austenite may not be sufficiently refined. This is because there is a risk of becoming a mixed grain structure. In particular, when considering improvement in toughness of the steel sheet, it is more preferable to perform rolling at a cumulative reduction of 50% or more in the austenite recrystallization region and a cumulative reduction of 30% or more in the non-recrystallization region.

  In the two-phase rolling in (a) above, an appropriate amount of dislocation is introduced into the ferrite structure of the steel sheet, the yield stress at room temperature of the steel sheet is increased by strengthening the dislocation, and the yield stress of the steel sheet during heating during thermal processing is This is an important process for easily softening and lowering the dislocations by eliminating and rearranging the dislocations.

  In the temperature range of the two-phase rolling, if the rolling is performed in a temperature range where the ferrite fraction is excessively small, the introduction of dislocations into the ferrite that transforms in the cooling process after the end of rolling becomes insufficient, so the ferrite fraction becomes 20% or more. Roll in the temperature range. If the ferrite region has a temperature range of 20% or more, ferrite transformation during rolling can be expected with certainty, and work dislocations can be introduced into most of the structure.

  Cumulative rolling reduction in two-phase rolling introduces work dislocations into the ferrite structure of the steel sheet, maintains the yield stress at room temperature of the steel sheet, and ensures a decrease in the yield stress of the steel sheet during heating during thermal processing. Therefore, it is necessary to make it 10% or more. On the other hand, when the cumulative rolling reduction in the two-phase rolling exceeds 75%, the ferrite into which the work dislocation has been introduced is recrystallized, conversely, the dislocation density is lowered, and the yield stress of the steel sheet during heating during the heat working is reduced. This is not preferable because a sufficient decrease in the thickness cannot be expected, and the anisotropy of the material increases.

  In the present invention, the two-phase region rolling strictly includes two-phase region to ferrite region rolling, but for convenience, it is referred to as two-phase region rolling including ferrite region rolling. That is, the effect of the steel sheet of the present invention can be achieved by introducing dislocations into ferrite, and the same effect can be obtained even when rolling is performed in a ferrite region where transformation is completed.

  Moreover, when it is desired to increase the productivity of the two-phase region rolling, it is preferable to set the rolling start temperature of the two-phase region rolling to more than 650 ° C. and not more than 700 ° C. In addition, when it is set as such a comparatively high temperature range of two phases, recrystallization occurs, rather it causes a decrease in dislocation density, or it becomes easier to form a cell structure, and a steel sheet during heating in a heat processing operation Since the yield stress may be insufficiently reduced, the upper limit of the cumulative rolling reduction in the two-phase rolling is limited to 30%, and is preferably set to 10 to 30%.

  Further, in order to sufficiently introduce the dislocation density in the above-mentioned two-phase region rolling and further increase the amount of reduction in the steel sheet yield stress during heating in the heat processing operation, the rolling start temperature of the two-phase region rolling exceeds 500 ° C., 650 It is preferable that the temperature is not higher than ° C. and the cumulative rolling reduction is 10 to 70%. If the rolling start temperature of the two-phase region rolling is more than 500 ° C. and 650 ° C. or less, the formation of recrystallization and cell structure is suppressed, so that the cumulative rolling reduction can be increased. However, when the cumulative rolling reduction exceeds 70%, depending on the two-phase region rolling temperature, recrystallization and the formation of a cell structure may occur. Therefore, it is preferable to limit the upper limit of the cumulative rolling reduction to 70%.

  In the two-phase rolling of (a), it is preferable that the rolling reduction per pass of each rolling pass is 15% or less as shown in (b). That is, when the rolling temperature in the two-phase rolling is high, or / and when the cumulative rolling reduction is high, if the rolling reduction per pass of each rolling pass is more than 15%, recrystallization is likely to occur. Since the increase in dislocation density may not be achieved effectively, the upper limit of the rolling reduction of each pass is preferably 15%.

  In addition, after the completion of the two-phase region rolling of (a), a step of accelerated cooling to a temperature range of 400 ° C. or lower at a cooling rate of 3 to 100 ° C./s as shown in (c) above. It is preferable to add.

  That is, accelerated cooling from the two-phase region to the ferrite region after the end of the two-phase region rolling to the above temperature region introduces thermal strain caused by the transformation dislocation and the temperature difference between the surface and the inside, and ferrite. The dislocation density therein can be further increased.

  If the cooling rate in this accelerated cooling is less than 3 ° C./s, the rapid cooling effect cannot be obtained and the introduction of dislocations is not sufficient, so that it is preferably 3 ° C./s or more. On the other hand, when the cooling rate exceeds 100 ° C./s, the quenching effect is saturated, and it is not easy to industrially cool at 100 ° C./s. Therefore, in the present invention, the upper limit of the accelerated cooling rate is 100 ° C./s. It is preferable to use s. However, even if the cooling rate exceeds 100 ° C./s, the quenching effect is only saturated and the effect is not reduced.

  In addition, the cooling stop temperature range in the accelerated cooling is set to 400 ° C. or lower in order to prevent the dislocation density from being reduced or rearranged even when air-cooled to room temperature after accelerated cooling. Is preferred.

  The production methods (a) to (c) described above are methods in which work dislocations are introduced into the ferrite of the steel sheet mainly by two-phase rolling, but as shown in the above (d), the ferrite is mainly produced by accelerated cooling after rolling. It is also possible to introduce dislocations into

  The method of introducing appropriate dislocations into the ferrite of the steel sheet by the accelerated cooling step after rolling without using the two-phase region rolling shown in (d) above is the two-phase region shown in (a) to (c) above. Compared with the method of introducing work dislocations into the ferrite of a steel sheet by rolling, it is possible to suppress a decrease in productivity, an increase in rolling load, a deterioration in rolling shape, and the like.

  The heating temperature of the steel slab in (d) and the reason for limitation are the same as in the manufacturing method in (a).

  The austenite region rolling in the above (d) is limited in order to refine the austenite grain size before ferrite transformation and refine the ferrite transformation structure in order to ensure the toughness of the steel sheet, as in the above (a). If the cumulative rolling reduction in austenite zone rolling is less than 50%, the austenite becomes insufficiently refined depending on the rolling temperature zone, so this cumulative rolling reduction is set to 50% or more.

  Further, in the austenite region rolling, it is more preferable for the austenite grain size refinement to include a cumulative rolling reduction at 1000 ° C. or less of 30% or more.

  The accelerated cooling after the hot rolling in (d) is performed in order to introduce dislocations into the ferrite of the steel sheet. The cooling start temperature range in the accelerated cooling needs to be a temperature range where the ferrite fraction is 50% or more and a temperature range of 500 ° C. or more. When accelerated cooling is started from a temperature range where the ferrite fraction is less than 50%, bainite or martensite transformed at a low temperature during accelerated cooling becomes the main structure, and ferrite into which the desired dislocation is introduced does not become the main structure. . Bainite or martensite in the steel sheet can sufficiently increase the yield stress of the steel sheet at room temperature, but the decrease in the yield stress of the steel sheet at 500 to 600 ° C. is not sufficient. It is not preferable.

  On the other hand, when the cooling start temperature range in accelerated cooling is less than 500 ° C., dislocations are not sufficiently introduced into the ferrite of the steel sheet, the yield stress of the steel sheet at room temperature cannot be secured, and the yield of the steel sheet during heating in the heat processing operation Since the stress is not sufficiently lowered, it is not preferable.

Moreover, the cooling rate and cooling stop temperature range in accelerated cooling are limited to 400 ° C. or less at 3 to 100 ° C./s for the same reason as in the above (c).
The above is the reason for limiting the microstructure and manufacturing method of the steel sheet in the present invention. In general, as a method for limiting the strength of a steel sheet within a predetermined range, there are control of the amount of added element and control of the microstructure or manufacturing process control for obtaining the microstructure, but the manufacturing cost of the steel sheet is not necessarily the same in both methods. It does not always become. In general, two-phase rolling has poor steel plate production efficiency, and it is desirable to avoid it from the viewpoint of cost. However, since there is a merit that the steel plate strength decreases during heating during heat processing by introducing dislocations, and the efficiency becomes higher during heat processing, it is difficult for those skilled in the art to determine which method is more economical as a whole. Depending on the situation, it is up to the skilled person to decide which method to choose.

Example 1
This example relates to the case where the maximum surface heating temperature is 850 ° C.

  First, a description will be given of a steel plate subjected to a heat processing operation.

  Table 1 shows steel plate components, and Table 2 shows steel plate production conditions. As shown in Table 2, steel plates produced under different production conditions for the same steel plate components are also prepared. In addition, the plate thickness before rolling of the steel plate is all 150 mm, and the slab heating temperature is 1250 ° C., which is also common for each condition.

  Each steel plate was finished into a steel plate having a thickness of 18 mm under the steel plate production conditions shown in Table 2. Steel plates within the scope of the present invention are steel plate numbers A1 to A14, and steel plates outside the scope of the present invention, that is, conventional steel plates are B1 to B7. Table 2 also lists the tensile strength and yield strength of the steel plate and the value of vTrs in the Charpy test of the steel plate. The steel sheets A1 to A14 in Table 2 are characterized in that the yield strength and tensile strength are all within the range of the examples of the present invention, and the strength is lower than that of B1 to B7, which are conventional steel sheets.

  Table 2 also shows the microstructure of the steel sheet. The distinction between ferrite, pearlite, and bainite was made by microstructural observation using an ordinary optical microscope, but it is impossible to determine whether or not dislocations are introduced into ferrite by optical microscope observation. A distinction was made between introduced ferrite and ferrite without dislocation introduction. As can be seen from Table 2, it can be seen that the steel sheet produced by carrying out the two-phase region rolling contains sufficient dislocation-introducing ferrite.

  Table 2 also shows the Charpy test results. In the present invention, the Charpy test is not particularly required. However, there is a case where Charpy characteristics are required, and there may be a case where it is necessary to meet the industrial demand for a steel sheet with excellent heat workability and a steel sheet with excellent Charpy characteristics. In this case, it is necessary to secure the Charpy characteristics while suppressing the strength, but as can be seen from Table 2, in such a case, two-phase rolling and accelerated cooling may be performed at the time of manufacturing the steel sheet. However, since two-phase rolling and accelerated cooling increase the steel sheet manufacturing cost, those skilled in the art should select a steel plate manufactured by two-phase rolling and accelerated cooling while considering the characteristics required for the steel plate. You just have to decide.

  The steel plate of Table 2 was processed into a rectangle having a width of 500 mm and a length of 1000 mm, and gas burner heating and water cooling were performed. Gas burner heating was performed under the conditions shown in Table 3, and the maximum surface heating temperature was adjusted by the gas burner heating rate. Water cooling was performed 10 cm behind the heated portion, and the water cooling torch was moved at the same speed as the gas burner. As for the maximum heating temperature, a drill hole having a diameter of 3 mm was formed by machining on the back surface of the steel sheet, and the maximum heating temperature of the surface was measured by inserting a thermocouple therein. FIG. 5 is a conceptual diagram illustrating the thermal processing procedure. As shown in FIG. 5 (a), linear heating was performed at three locations from a 1000 mm side in a direction perpendicular to the side. The hatched portion in FIG. 5 (a) is a portion subjected to thermal processing. The length of the heated part is 200 mm. FIG. 5B is a conceptual diagram illustrating the thermal processing operation. Although this figure is a figure when the heat processing operation is carried out on the part surrounded by the curve in FIG. 5A, it was similarly carried out on the other parts. The work was performed by moving the gas burner torch 6 at a constant speed and moving the gas burner torch 7 to follow it. The moving speed of the water-cooled torch and the gas burner torch is the same.

  Heating and cooling were performed 5 times for one place. That is, the hatched portion in FIG. 5A was heated and cooled five times. Thereafter, the width is measured and the difference from the initial width before heating is calculated, whereby the amount of shrinkage (lateral shrinkage in this case) generated by heating and cooling can be determined. FIG. 5 (c) also describes the amount of lateral contraction. A width for measurement is determined before heating, the width is measured, heated and cooled five times, and the width is measured again. When those values 8 are W1 and W2, the difference between them, that is, W = (W1−W2) corresponds to the lateral contraction amount. In addition, this W is not a value measured for all six places subjected to the thermal processing shown in FIG. 5A, but a value measured for a place surrounded by a curve in FIG. It was defined as the amount of lateral shrinkage in the steel sheet and its thermal processing conditions.

  Table 4 shows the amount of lateral shrinkage of each steel plate when the maximum heating temperature of the surface is 850 ° C. Test numbers of SA1-14 are examples of the present invention, and SB1-7 are comparative examples. From Table 4, the test numbers SA1 to 14 within the scope of the present invention all have a lateral shrinkage amount exceeding 0.5 mm, whereas the comparative examples SB1 to 7 have the maximum lateral shrinkage amount. 0.40 mm, which is smaller than the example of the present invention. These lateral shrinkage amounts are the results obtained under the same heating and cooling conditions, i.e., the same thermal processing construction, and if the thermal processing work is carried out using a steel sheet within the scope of the present invention, it is more than the prior art. It can be seen that high work efficiency can be obtained.

  When the results in Table 4 are compared in more detail, the following can be understood.

  That is, as shown in Table 2, test numbers SA9 to SA14 are examples in which two-phase region rolling was performed and a steel sheet containing dislocation-introducing ferrite was used. These steel sheets have high room temperature strength because dislocations are introduced into the ferrite. On the other hand, dislocations tend to disappear when heated, so the strength during heating tends to be low. The effect can also be read from the results in Table 4. For example, SA7 and SA14 are the cases where steel plates A7 and A14 are used, and the strength levels of these steel plates are substantially equal. However, the amount of lateral shrinkage, that is, the thermal processing characteristics, was greater for SA14 than for SA7. This is presumably because the decrease in steel sheet strength during heating was greater for SA14.

  Thus, even when the same room temperature strength level is used, thermal processing characteristics are superior when two-phase rolling is performed. However, since the two-phase rolling increases the manufacturing cost of the steel sheet, a person skilled in the art may determine which is better overall by comparing them.

Table 4 also shows the heat processing time required for heating and cooling after 5 times of cooling and cooling until the radius of curvature reaches 4.0 m. In the thermal processing method shown in FIG. 5, the radius of curvature can be defined in two directions as shown in FIG. 3, but in this embodiment, the average value of these radii is the radius of curvature after the thermal processing. Defined.
As can be seen from Table 4, in the examples of the present invention, all are less than 150 minutes, which is 30% or more compared with the comparative example, and in some cases, it may be 50% (halved). From this result, the high effect of the present invention can be understood.

(Example 2)
This example relates to the case where the maximum surface heating temperature is 700 ° C.

  The thermal processing work conditions were almost the same as in Example 1, but only the heating rate was changed in order to set the maximum surface heating temperature to 700 ° C.

  Table 5 shows the amount of lateral shrinkage after thermal processing of each steel plate. In the examples of the present invention, the amount of lateral shrinkage exceeded 0.30 mm, whereas in the comparative example, the maximum was 0.22 mm. That is, even when the maximum surface heating temperature is 700 ° C., when a steel plate within the scope of the present invention is used, the heat processing operation can be performed efficiently. In addition, when the comparative example of Table 4 and the example of this invention of Table 5 are compared, the comparative example of Table 4 also has the same amount of lateral shrinkage as the example of this invention of Table 5. This is because the maximum heating temperature is different, and it seems that a higher maximum heating temperature is better for improving the efficiency of the thermal processing work. However, in Table 5, since the maximum heating temperature is lower than that in Table 4, the heating rate is faster in Table 5 on the contrary. For this reason, the working time required to obtain the same lateral contraction is shorter under the conditions shown in Table 5. For this reason, the maximum heating temperature of 700 ° C. is not necessarily inferior to the 850 ° C. condition. Which one is more efficient may be determined by a person skilled in the art in consideration of the work environment and the like.

  Moreover, TA9-14 is a case where the steel plate which implemented the two-phase area rolling is used, but, like Example 1, the heat processing characteristic is more excellent when the two-phase area rolling is performed even at the same strength level. I can understand the tendency. However, since the point of increasing the steel sheet manufacturing cost is the same as that of Example 1, it should be determined by those skilled in the art whether or not the thermal processing characteristics are further improved even if the two-phase rolling is performed.

  As in Example 1, heating and cooling were further performed after heating and cooling five times, and thermal processing was performed until the radius of curvature reached 4.0 m. And the time required for heating and cooling work was measured. As in Example 1, all of the examples of the present invention were less than 120 minutes, and the effectiveness of the present invention can be understood compared to the comparative example.

(Example 3)
This example relates to the case where the maximum heating temperature is 600 ° C., and only the heating rate is different from those in Examples 1 and 2.

  Table 6 shows the amount of lateral shrinkage. Also in the case of this example, in the case of the example of the present invention, all the lateral shrinkage exceeded 0.20 mm, whereas in the comparative example, the maximum was 0.16 mm. In Example 3, since the maximum heating temperature is 600 ° C., the amount of lateral shrinkage after 5 times of water heating and cooling is smaller than in Examples 1 and 2, but the heating rate can be increased more than that, Efficiency as a heat processing operation is not necessarily low. As in Examples 1 and 2, heating and cooling were further performed after heating and cooling five times, and thermal processing was performed until the curvature radius reached 4.0 m. And the time required for heating and cooling work was measured. Similarly to Example 1, all of the examples of the present invention were less than 110 minutes, and the high effect of the present invention can be understood compared to the comparative example. In addition, when the surface maximum heating temperature is lowered, the heating speed can be increased accordingly. Compared with Examples 1 and 2, the case of Example 3 has a tendency to obtain a curvature of 4.0 m in the shortest time. On the other hand, since the number of operations increases, a person skilled in the art may select which construction condition is performed as necessary. In any construction condition, when a steel plate within the scope of the present invention is used, a heat processing operation that is more efficient than a conventional steel plate is possible.

  In Table 6, Comparative Example UB8 uses a steel plate B8 in which the yield strength of the steel plate is within the range of the present invention, but the tensile strength of the steel plate is outside the range of the present invention. In this case, as shown in Table 6, the amount of lateral shrinkage of the heated portion reached the same amount of lateral shrinkage as that of the example of the present invention, but the time until the radius of curvature was 4 m or less was shorter than that of the example of the present invention. It was not so short. The reason for this is that although the initial lateral shrinkage amount was large, the thermal processing efficiency was subsequently lowered, and as a result, it was necessary to repeat the thermal processing operation many times until the radius of curvature reached 4 m. Thus, it can be understood that not only the yield strength of the steel sheet but also the tensile strength is important in the thermal processing work with a large processing amount that is handled by the present invention.

Example 4
In this example, the maximum surface temperature is 1000 ° C. In the present invention, since the upper limit of the maximum temperature reached is 900 ° C., all shown in Example 4 are comparative examples. For test numbers VA1 to VA14, the steel sheet itself is in the range of the present invention, but the highest achieved temperature is 1000 ° C, so that it is a comparative example, and for test numbers VB1 to VB7, the steel sheet itself is in the range of the present invention. It is something that is outside. Table 7 shows the results. First, it can be seen from Table 7 that it is necessary to extremely slow the heating rate in order to set the maximum surface temperature to 1000 ° C. Even when compared with the case where the maximum temperature in Table 4 was 850 ° C., the heating rate had to be reduced from 10 cm / min to 4 cm / min. Certainly, the amount of lateral shrinkage after heating five times is greater when the maximum temperature shown in Table 7 is 1000 ° C. than when the maximum temperature shown in Table 4 is 850 ° C. That is, the amount of lateral contraction increases under the condition that the number of times of heating is the same. In this sense, the higher the maximum temperature reached, the more shrinkage can be obtained. However, an object of the present invention is to improve the efficiency of the thermal processing method. Even if the amount of shrinkage increases, if the heating takes longer than that, the efficiency of the thermal processing operation becomes worse. Actually, as shown in Table 7, the time until the radius of curvature becomes 4 m or less exceeds 200 minutes, and the time required for the comparative examples shown in Tables 4, 5, and 6 is required. It can be seen that it is. This is due to the low heating rate. For this reason, in the present invention, the upper limit of the maximum temperature reached is set to 900 ° C.

It is a conceptual diagram explaining two deformation modes which generate | occur | produce in the part which implemented the heat processing operation | work, lateral contraction, and an angular deformation. It is the conceptual diagram explaining the method of generating an angular deformation in a heating part and giving a curvature to a steel plate. It is the conceptual diagram explaining the method of generating transverse shrinkage in a heating part and giving a curvature to a steel plate. It is the figure which showed the relationship between the number of times of heating and cooling, and the steel plate yield strength. It is a conceptual diagram explaining the heat processing work method used in the Example of this invention, and the amount of lateral shrinkage which arose in the heating part.

Explanation of symbols

1, 2: Sides of steel plate 3: Linear heating region 4: Direction defining the radius of curvature after heat processing 5: Transverse shrinkage generated on sides 1, 2 of the steel plate by heat processing 6: Gas burner torch 7: Water-cooled torch 8: Width before thermal processing (W1) and Width after thermal processing (W2)
9: Steel plate 10: Lateral shrinkage 11: Radius of curvature

Claims (5)

The front or back of the steel plate by a gas burner heating a linear, subsequently in a thermal processing method of the processing to the steel sheet so that a maximum radius of curvature of the heating unit by causing bending deformation of the steel sheet by water cooling becomes less 4.0 m, as the steel sheet, by mass%, C: 0.001~ 0.048%, Si: 0.005~0.4%, Mn: 0.05~0.6%, Al: 0.002~0.12 %, N: 0.001 to 0.01%, P: 0.03% or less, S: 0.01% or less are limited, the balance is made of iron and inevitable impurities, and the yield strength at room temperature is 200. a ～300MPa, and using a steel tensile strength of 250~400MPa at room temperature, in a vertical direction with respect to one of the sides of the steel plate, and the surface or area on the back surface of the steel sheet except for the central portion, Maximum temperature of the heating unit By linear heating to 900 ° C. or less, a plurality of linear heating regions are formed at intervals along the side, and the linear heating region shrinks and deforms in a direction along the side. A method for thermal processing of a steel sheet, comprising bending and deforming the steel sheet.
  2. The method for heat-processing a steel sheet according to claim 1, wherein the steel sheet contains 20 to 95% of a ferrite phase in which dislocations are introduced by processing or transformation strain in the microstructure.   The steel sheet is further, in mass%, Ti: 0.0005 to 0.02%, V: 0.005 to 0.1%, Nb: 0.0005 to 0.05%, and Mo: 0.05. It contains 1 type or 2 types or more of -0.3%, The heat processing method of the steel plate of Claim 1 or 2 characterized by the above-mentioned.   The steel sheet further contains 0.05% to 0.3% of Ni, Cu, and Cr in a total amount of one or more of Ni, Cu, and Cr. 4. A method for thermally processing a steel sheet according to any one of 3 above.   The said steel plate contains 0.0005-0.03% of 1 type (s) or 2 types or more of Mg, Ca, and REM in the total amount further by the mass%. 5. A method for thermally processing a steel sheet according to any one of 4 above.

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