JP6409848B2 - Steel slab joining method, continuous hot rolling method, and hot rolled steel sheet manufacturing method in continuous hot rolling - Google Patents

Description

  The present invention relates to a method for joining a preceding steel slab and a subsequent steel slab on the entry side of a finishing mill in a continuous hot rolling line in continuous hot rolling.

  Conventionally, as a method of joining the preceding steel slab and the subsequent steel slab at the entry side of the finish rolling mill of the continuous hot rolling line, for example, The tail end and the tip of the subsequent billet are placed in contact with each other in a non-contact manner, and in this state, the tail end of the preceding billet and the tip of the following billet are rapidly heated by the induction heating device, and then heated. In addition, there is known a method in which the tail end of the preceding steel piece and the tip of the subsequent steel piece are brought into contact with each other and pressed (upset).

  However, when joining in an air atmosphere, an oxide having a melting point higher than the melting point of the steel (1400 to 1600 ° C.) such as Cr, Ti, Mn, Al, Si, etc. in the steel (for example, Cr oxide) : For stainless steels and high-strength steels that contain components that generate a melting point of about 2000 ° C., these oxides generated on the joint surface during induction heating remain in the joint as a solid phase even after upsetting. The strength is remarkably reduced, and problems such as breakage of the joint portion occur in finish rolling in the subsequent process.

  In order to solve this problem, a method has been proposed in which the temperature of each joint surface of the preceding steel slab and the subsequent steel slab is set to be equal to or higher than the liquidus temperature of the steel slab by using an induction heating device in the heating process (Patent Document 1). reference).

  However, in the heating by the induction heating device, since the generated induced current does not easily flow at the corner portion (width end portion) of the steel slab, the temperature of the portion to be joined (temperature in the width direction of the steel slab) is changed to the width end portion. The closer the temperature is, the lower the temperature rise, and there is a problem that the part to be joined cannot be joined over the entire area when the steel piece is pressed. Here, the width direction is a direction perpendicular to the rolling direction in a horizontal plane.

  With respect to this problem, a method of accelerating the temperature rise of the width end portion by arranging a magnetic body within a predetermined range from the width end portion to heat and raise the temperature of the steel piece (see Patent Document 2), After rapidly heating the tail end and the tip of the succeeding steel slab, and then butting the tail end of the heated preceding steel slab and the tip of the succeeding steel slab, the induction coil is connected to the end of the joint in the plate width direction. Plate material joining method (refer to Patent Document 3), which is performed by applying pressure again while heating by an eddy current that circulates in the plate width direction endmost part, and after the preceding steel piece At least one of the end portion and the tip end portion of the following steel slab is formed into a convex shape, and both steel pieces are brought into contact with each other at the center in the width direction, and the thickness of the steel slab at the center in the width direction. Applying an alternating magnetic field penetrating in the direction and pressing the steel pieces together while heating by the induced current induced by this alternating magnetic field The Rukoto, a method of sequentially bonding toward the billet central portion to the width end (see Patent Document 4) it has been proposed.

  By using both the method for promoting the temperature rise at the width end portion and the method for setting the temperature of each joint surface to be higher than the liquidus temperature of the steel slab, an oxide having a melting point higher than that of the steel can be obtained. It is considered that even the steel slab containing the component to be generated can improve the bonding strength of the width end portion.

  However, in the methods described in Patent Documents 2 to 4, a device for heating the width end portion is required, and there has been a demand for a method that can more easily prevent breakage of the joint portion.

JP 2000-271605 A JP-A-8-001202 JP 7-001006 A JP-A-8-141602

  The present invention has been made to solve the above-described problems, and does not require an apparatus used only for heating the width end portion, and can easily prevent fracture of the joint portion in the finish rolling in the subsequent process. An object of the present invention is to provide a method for joining steel slabs in continuous hot rolling, a continuous hot rolling method using the joining method, and a method for producing a hot-rolled steel sheet using the continuous hot rolling method.

[1] The tail end of the preceding steel slab and the tip of the succeeding steel slab are arranged so as to face each other in a non-contact manner on the entry side of the finish rolling mill of the continuous hot rolling line. A steel slab in continuous hot rolling, comprising: a heating step for heating the tip of the steel slab; and a joining step for pressing and joining the tail end of the heated preceding steel slab and the tip of the succeeding steel slab In the joining method of
In the heating step, heating is performed until the temperature in the range of 50 to 100% of the total width of the joining surface at the tail end of the preceding steel slab and the tip of the subsequent steel slab becomes equal to or higher than the liquidus temperature of the steel slab. And
And the amount of upset in the joining step is 1.1 to 7.0 times the sum of the maximum melting depth of the preceding steel slab and the maximum melting depth of the succeeding steel slab. A method of joining steel slabs in continuous hot rolling.
[2] A continuous hot rolling method using the joining method according to [1].
[3] A method for producing a hot-rolled steel sheet, wherein the continuous hot rolling method according to [2] is used.

  Here, as shown in FIG. 7, the melting depth is the length in the rolling direction of each steel slab in the range of the liquidus temperature or higher due to heating, and the maximum melting depth is shown in FIG. As shown, it is the maximum value of the melting depth in the entire joining surface (full width and full thickness). Depending on the heating conditions, the maximum melting depth may be the maximum at the center of the plate thickness, or the upper surface or the lower surface may be the maximum.

The amount of upset is the amount of steel slab push in the rolling direction in the joining process (the distance between the joining surface at the tail end of the preceding steel slab to be joined and the joining surface at the tip of the succeeding steel slab is zero) (The amount of push-in from the state in which the two surfaces are closely aligned)).
The amount of upset depends on the relative distance between the preceding steel slab and the succeeding steel slab, and is therefore the sum of the pushing amounts of the preceding steel slab and the subsequent steel slab. If it is the conditions which become the value within the range of the upset amount, the pushing amount of each of the preceding steel slab and the succeeding steel slab can be arbitrarily determined. For example, only the subsequent billet may be pushed in a desired upset amount without pushing (moving) the preceding billet.

  According to the present invention, an apparatus used only for heating the width end portion is not required, and it is possible to easily prevent the joint portion from being broken by finish rolling in a subsequent process.

It is the schematic which shows the equipment arrangement | sequence from the coil box of a continuous hot rolling line to the 1st stand of a finishing mill. It is a schematic sectional drawing of a joining apparatus. It is the schematic of an induction heating apparatus. It is explanatory drawing for demonstrating the flow of an alternating magnetic field and an induced current. It is a graph which shows the temperature distribution of a joining interface. When input power is 5200 kW, it is a graph which shows an example of a mode that the heating time required for 50% melting | fusing of a full width changes according to induction heating frequency (kHz). It is a figure for demonstrating the definition of a melting depth. It is a figure for demonstrating the definition of the maximum melting depth. It is a graph which shows distribution of fusion depth. It is a graph which shows distribution of the joint part hot tensile strength of a junction part. It is a graph which shows the relationship between the amount of upset / maximum fusion depth, and junction strength. It is a graph which shows the ratio with respect to the full width in the joining surface of the area | region heated more than the liquidus temperature of steel, and the amount of upset / maximum melt depth.

  Hereinafter, the present invention will be described with reference to the drawings. In addition, this invention is not limited by this embodiment.

  FIG. 1 is a schematic diagram showing an equipment arrangement from a coil box of a continuous hot rolling line to a first stand of a finishing mill. In FIG. 1, reference numeral 1 is a coil box that winds a plate material from a rough rolling mill, and reference numeral 2 is a crop that cuts the tail end of a preceding steel slab S1 and the tip of a subsequent steel slab S2 that are unwound from the coil box 1. Shear, code 3 is a joining device for joining the cut surfaces (joint surfaces) of the preceding steel piece S1 and the succeeding steel piece S2, reference numeral 4 is a leveler, reference numerals 5a to 5c are pinch rolls, reference numeral 6 is a descaling device, reference numeral Reference numeral 7 denotes a first stand of a finishing mill.

  FIG. 2 is a schematic cross-sectional view of the bonding apparatus 3. As shown in FIG. 2, the joining device 3 has the joining surfaces of the cutting ends of the preceding steel piece S <b> 1 in which the rear end crop is cut off by the crop shear 2 and the succeeding steel piece S <b> 2 in which the front end crop is cut off. Induction heating that heats the left and right clamping devices 8 and 9 that are gripped so as to face each other in a non-contact manner, and the cutting ends of the preceding steel piece S1 and the subsequent steel piece S2 that are held by the clamping devices 8 and 9 The apparatus 10 and the clamping device 8 are pressed and moved to the clamping device 9 side, and the joining surfaces of the cutting ends of the preceding steel piece S1 and the succeeding steel piece S2 heated by the induction heating device 10 are brought into contact with each other and upset joined. And a press cylinder 11 that forms a joint portion α, and a mistake prevention plate 20 that prevents the preceding steel piece S1 and the subsequent steel piece S2 from shifting in the vertical direction during the upset joining.

  The joining device 3 is installed on a carriage 17 that can travel on a rail 19 (see also FIG. 1) that extends a predetermined length along the line direction. Further, the billet transfer table roller 18 installed in the travelable range of the carriage 17 is a liftable table roller, and the carriage table roller 18 corresponding to the position of the joining device 3 is pushed down by the carriage 17. It has become. By setting the joining apparatus 3 to such a configuration, the preceding steel piece S1 and the succeeding steel piece S2 can be joined without stopping the conveyance of the steel piece.

  FIG. 3 is a schematic diagram of the induction heating apparatus 10. The induction heating device (high frequency induction heating device) 10 is for penetrating an alternating magnetic field in the plate thickness direction of each cutting end of the preceding steel piece S1 and the subsequent steel piece S2. As shown in FIG. 3, the induction heating device 10 includes a pair of magnetic pole cores 13 disposed above and below the cutting ends of the preceding steel piece S1 and the succeeding steel piece S2, and the magnetic pole cores 13 in the vertical direction. A coil 14 continuously wound and a power source 15 are provided.

  FIG. 4 is an explanatory diagram for explaining the flow of the alternating magnetic field and the induced current. By using the induction heating device 10 having the above-described configuration, as shown in FIG. 4, by passing an alternating magnetic field in the thickness direction of each cutting end of the preceding steel piece S1 and the succeeding steel piece S2, each cutting end An eddy current is generated to preferentially heat the joint surfaces. In the present embodiment, the so-called transverse joining device 3 that synchronizes the heating / joining process with the travel of the steel slab is adopted, but when performing the heating / joining process with the joining device 3 stopped, A looper 16 indicated by a broken line in FIG. 1 is used.

Here, in this embodiment, the steel types of the preceding steel slab S1 and the succeeding steel slab S2 are not particularly limited. For example, at least one of the preceding steel slab S1 and the succeeding steel slab S2 has a melting point of steel (1400). It is possible to obtain a steel type containing 1% by mass or more of an element that generates an oxide having a melting point higher than ˜1600 ° C.). An oxide having a melting point higher than that of steel is an oxide such as Cr, Ti, Mn, Al, Si or the like (specifically, Cr oxide: melting point of about 2000 ° C., other TiO ₂ , MnO, Al ₂ O ₃ , SiO _2, etc.).

  The method for joining steel slabs according to the present embodiment is such that the tail end of the preceding steel slab S1 and the tip of the subsequent steel slab S2 face each other in a non-contact manner on the entry side of the above-described finishing rolling mill of the continuous hot rolling line. The heating step of heating the tail end of the preceding steel slab S1 and the tip of the succeeding steel slab S2, and the joining that presses and joins the tail end of the heated preceding steel slab S1 and the tip of the succeeding steel slab S2 Process.

  In the heating step, the induction heating device 10 is used, and the temperature in the range of 50 to 100% of the total width of each joining surface of the preceding steel slab S1 and the subsequent steel slab S2 is equal to or higher than the liquidus temperature of the steel slab. It heats until it becomes. In the joining step, the upset amount is 1.1 to 7.0 times the sum of the maximum melting depth of the preceding steel slab S1 and the maximum melting depth of the succeeding steel slab S2. .

  Hereinafter, these features in the heating step and the bonding step will be described in detail.

  First, the characteristics of the heating process will be described.

  FIG. 5 shows the length direction of a steel slab when heating of 1.7% Si steel is completed when the heating time is 3 seconds, 4 seconds, 5 seconds, or 6 seconds and the other conditions (heat input) are the same. Shows the temperature distribution.

  Here, the temperature distribution shown in FIG. 5 is a temperature distribution at the t / 2 position from the surface of the steel sheet in the thickness direction, where t is the total thickness of the steel sheet (steel piece). The plate width is 1000 mm. As shown in FIG. 5, the temperature at the joint interface tends to be equal to or higher than the liquidus temperature near the center of the width, but the end of the width is lower than the liquidus temperature. In the portion where the temperature of the bonding interface is lower than the liquidus temperature, Si oxide having a melting point higher than the melting point of steel remains as a solid phase at the bonding interface after upsetting, and the bonding strength is significantly reduced to become an unbonded portion.

  As in the case where the heating time in FIG. 5 is 3 seconds, when the ratio of the unjoined portion (the portion not higher than the liquidus temperature) of the width end to the full width is large (above the liquidus temperature of the steel) When the heating area is less than 50% of the plate width), tension is concentrated on the joint (part heated to the liquidus temperature or higher) in the subsequent finishing rolling, and the joint can be separated. There is sex. On the other hand, for example, when the heating time is longer and the region heated to the temperature higher than the liquidus temperature of steel is 50% or more with respect to the entire width, the ratio of the unbonded portion is low, so that the bonding process is described later. In this case, the upset amount is 1.1 to 7.0 times the maximum melting depth, so that, for example, even if rolling is performed to a plate thickness of 2 mm by a finishing mill consisting of 7 stands, the joint portion does not separate. Good continuous rolling can be continued.

  In the present embodiment, the region that is equal to or higher than the liquidus temperature of steel can be calculated by electromagnetic-heat conduction coupled finite element analysis. This electromagnetic-heat conduction coupled finite element method analysis uses general-purpose calculation software JMAG to model the target sheet bar, coil, and magnetic pole core, and the dimensions of the sheet bar, the gap between the joint surfaces, the electrical resistance of the steel, and the specific heat. , Thermal conductivity, density and relative permeability, coil / pole core dimensions and relative positional relationship with the sheet bar, and heating conditions (heating time, input power and frequency) can be set as appropriate.

  Moreover, in this embodiment, until the temperature of the width range of 50 to 100% is equal to or higher than the liquidus temperature of the steel slab with respect to the full width of each joint surface at the tail end of the preceding steel slab and the tip of the subsequent steel slab. In order to heat, although the example which adjusts heating time as a heating condition was shown, it is not limited to this example, You may employ | adopt the method of adjusting input electric power, a frequency, etc.

  FIG. 6 is a graph showing an example of a state in which the heating time required for melting 50% of the full width changes according to the induction heating frequency (kHz) when the input power is 5200 kW. For example, when the input power is 5200 kW, if a heating time equal to or greater than the heating time shown in the graph of FIG. 6 is secured at each induction heating frequency (kHz), the width range where the temperature is equal to or higher than the liquidus relative to the full width is 50. ~ 100%.

  As described above, in the present invention, in the heating step, the temperature in the range of 50 to 100% of the total width of each joint surface at the tail end of the preceding steel slab S1 and the tip of the succeeding steel slab S2 is a liquidus line of the steel slab. Heat until above temperature. Preferably, the width range is 70 to 100%, more preferably 85 to 100%.

  Next, features of the joining process will be described.

  Reference is now made to FIGS. FIG. 7 is a diagram for explaining the definition of the melting depth, and FIG. 8 is a diagram for explaining the definition of the maximum melting depth. As shown in FIG. 7, the melting depth is the length in the rolling direction of each steel slab in the range of the liquidus temperature or higher due to heating, and the maximum melting depth is as shown in FIG. It is the maximum value of the melting depth in the entire joining surface (full width and full thickness). Depending on the heating conditions, the maximum melting depth may be the maximum at the center of the plate thickness, or the upper surface or the lower surface may be the maximum.

  In addition, the amount of upset is the pushing amount of the steel slab in the rolling direction in the joining process (the distance between the joining surface at the tail end of the preceding steel slab to be joined and the joining surface at the tip of the succeeding steel slab has become zero. It is the amount of pushing from the state (the state in which the two surfaces are in perfect alignment). The amount of upset depends on the relative distance between the preceding steel slab and the succeeding steel slab, and is therefore the sum of the pushing amounts of the preceding steel slab and the subsequent steel slab. If it is the conditions which become the value within the range of the upset amount, each pushing amount of a preceding steel slab and a succeeding steel slab can be determined arbitrarily. For example, only the subsequent billet may be pushed in a desired upset amount without pushing (moving) the preceding billet.

  The relationship between this upset amount and the sum of the maximum melting depth of the preceding steel slab S1 and the maximum melting depth of the succeeding steel slab S2 will be described in detail. FIG. 9 is a diagram showing the distribution of the melting depth at the completion of heating when heating is performed under the same conditions as those shown in FIG.

  As shown in FIG. 9, as the heating time increases, the melting depth increases, and the maximum melting depth when the heating time is 6 seconds is 12.4 mm. In FIG. 10, after heating with a heating time of 6 seconds in this way, the steel piece was joined with an upset amount of 12.4 mm (1.0 times the sum of the maximum melting depth), and upset. The joint strength at each position in the plate width direction when steel pieces are joined with an amount of 13.7 mm (about 1.1 times the sum of the maximum melting depths) is shown. As shown in FIG. 10, the present inventors have intensively studied paying attention to the fact that the joint strength greatly increases as the amount of upset increases.

  FIG. 11 shows the hot tensile strength (heating temperature: 1000 ° C.) of the joint at a location 350 mm from the center of the plate width in the joint having a plate width of 1200 mm. As is apparent from FIG. 11, by increasing the amount of upset to 1.1 times or more of the sum of the maximum melting depths, the joint strength is rapidly improved, and the joint strength is 80% or more of the strength of the base material. The present inventors have found that can be obtained. On the other hand, it has also been found that when the amount of upset exceeds 7.0 times the sum of the maximum melt depth, the strength is saturated and buckling is more likely to occur.

  Therefore, in the present invention, in the joining step, the amount of upset is 1.1 to 7.0 times the sum of the maximum melting depth of the preceding steel slab S1 and the maximum melting depth of the succeeding steel slab S2. Preferably, it is 1.3 to 7.0 times.

  In this embodiment, the upset amount is 1.1 to 7.0 times the maximum melting depth at the tail end of the preceding steel slab S1 and the tip of the succeeding steel slab S2 as described above mainly. The case where the tail end of the preceding steel slab and the tip of the succeeding steel slab are respectively moved by the upset amount has been described. However, since the upset depends on the relative distance between the preceding steel slab and the succeeding steel slab, 1.1 to 7.0 of the sum of the maximum melting depth of the preceding steel slab S1 and the maximum melting depth of the succeeding steel slab S2. If it is a double upset amount, for example, only the subsequent steel piece S2 may be moved while the preceding steel piece S1 is fixed.

  In the present embodiment, the pressing force in the joining process is not particularly limited.

  As described above, in the present invention, in the heating process, the temperature in the range of 50 to 100% with respect to the entire width of each joining surface of the preceding steel slab and the subsequent steel slab becomes equal to or higher than the liquidus temperature of the steel slab. And the amount of upset in the joining process is 1.1 to 7.0 times the sum of the maximum melting depth of the preceding steel slab and the maximum melting depth of the succeeding steel slab, An apparatus used only for heating the end portion is not required, and it is possible to easily prevent the joint portion from being broken by finish rolling in the subsequent process.

  Moreover, in the continuous hot rolling method of this invention, if the joining method in the continuous hot rolling mentioned above is used, about other conditions, a conventionally well-known condition is employ | adopted and it sets suitably according to a steel type and the shape of steel. be able to. Furthermore, the manufacturing method of the hot-rolled steel sheet of the present invention can adopt conventionally known conditions for other conditions if this continuous hot rolling method is used.

  In the above description, only 1.7% Si steel is taken as an example, but the steel composition is 1.2 mass% at the maximum in the case of C, 4.0 mass% at the maximum in Si, and 6. in the case of Mn. 2% by mass, Cr: 35.0% by mass, Ti: 0.5% by mass, Al: 0.5% by mass, P: 0.5% by mass, S: 0. 4 mass%, Ni 25.0 mass% maximum, Mo Mo maximum 1.0 mass%, V 0.5 mass% maximum Good continuous rolling can be continued up to a finished sheet thickness of 2 mm.

  Hereinafter, the present invention will be described based on examples.

  Sheet bars each having a width of 1200 mm and a thickness of 30 mm as the preceding steel piece and the subsequent steel piece were subjected to the continuous hot rolling line shown in FIG.

  No. shown in Table 1. 1-44, both the preceding steel slab and the succeeding steel slab have mass%, Si: 1.7%, C: 0.12%, Mn: 2.0%, Cr: 0.1%. And what has the steel composition which remainder consists of Fe and an unavoidable impurity was used.

  No. 2 shown in Table 2 In Nos. 45-55, the preceding steel slab is No. 1 above. In the same manner as in 1-44, steel having different components in the succeeding steel piece was used. The trailing steel slab has, in mass%, Si: 0.5%, C: 0.12%, Mn: 2.0%, Cr: 0.1%, and the balance from Fe and inevitable impurities A steel having the following steel composition was used.

  Then, after the joining surfaces of the preceding sheet bar and the succeeding sheet bar are opposed to each other with a gap of 5 mm in the joining device 3, each is heated by the induction heating device 10 (size in the width direction 1300 mm, size in the longitudinal direction 240 mm). The joining surface was heated. The heating conditions at this time are an input power of 1000 kW and a frequency of 1000 Hz.

  Tables 1 and 2 show the results of several conditions for the heating time and the amount of upset. Table 1 shows the results calculated by the electromagnetic-heat conduction coupled finite element method analysis for the region where the temperature is higher than the liquidus temperature of steel.

  No. shown in Table 1. In 1-44, the maximum melting depth and the amount of upset were the same values at the tail end of the preceding steel material and the tip end of the succeeding steel material, indicating values on one side.

  No. shown in Table 2 In 45-55, the maximum melting depth and the amount of upset may differ between the tail end of the preceding steel material and the tip end of the following steel material, and each value is shown.

In the electromagnetic-heat conduction coupled finite element method analysis, general-purpose calculation software JMAG is used to model a sheet bar, a coil, and a magnetic pole core. In this model, the size of the sheet bar and the gap between the joint surfaces are the same as those of the above-described embodiment, and the physical property values are No. 1 shown in Table 1. No. 1 to 44 preceding steel slab and trailing steel slab, No. 2 shown in Table 2. In the preceding steel slabs of 45 to 55, electric resistance: 125 μΩ · cm, specific heat: 350 J / kg / degC, thermal conductivity: 30 W / m / degC, density: 7850 kg / m ³ , and relative magnetic permeability: 8. No. 2 shown in Table 2 In the subsequent steel slabs of 45 to 55, electric resistance: 110 μΩ · cm, specific heat: 350 J / kg / degC, thermal conductivity: 32 W / m / degC, density: 7850 kg / m ³ , and relative magnetic permeability: 8. The dimensions of the coil and magnetic pole core and the relative positional relationship with the seat bar are the same as those of the joining machine used in this embodiment, and the heating conditions (input power and frequency) are the same as in this embodiment.

Subsequently, the joining surfaces were brought into contact with each other and pressed by the pressing cylinder 11 with a pressing force of 3.4 kg / mm ² to complete the joining.

  After the joining was completed, rolling was performed to a plate thickness of 2 mm using a 7 stand mill of a finish rolling mill.

  In the present invention examples (Nos. 1 to 25), it was possible to continue good continuous rolling without separation of the joints after rolling up to a plate thickness of 2 mm.

  On the other hand, in the comparative example (Nos. 34 to 43), the ratio of making the full width of the width range heated to the temperature higher than the liquidus temperature of the steel slab in the heating process is less than 50%. Separated.

  Moreover, in the comparative examples (Nos. 26 to 34, 40), in the joining process, the upset amount / (the sum of the maximum melting depths) was less than 1.1, so the joints were separated during the continuous rolling. .

  In the comparative example (No. 44), buckling occurred during the continuous rolling because the upset amount / (the sum of the maximum melting depths) exceeded 7.0 in the joining step.

The above results are shown in FIG. FIG. 12 shows whether or not finish rolling is possible with respect to the ratio of the total width of the region heated above the liquidus temperature of the steel and the ratio of the length in the rolling direction and the amount of upset in the range where the liquidus temperature is exceeded. It is. In FIG. 12, “◯”, “◎”, and “×” indicate the following.
◯: No breakage occurred even when the sheet was rolled up to a plate thickness of 2 mm by a 7 stand mill of a finishing mill and continuously joined and finish-rolled 10 times or more.
◎: JIS14A tensile test taken from the center of the width center plate thickness so that the condition of ○ is satisfied, the joint portion is at the center of the evaluation portion, and the axial direction of the test piece is positioned as the rolling direction. This means that the elongation at break was 80% or more compared to the normal part.
(Alternatively, this means that there was no defect in the joint.)
X: When the sheet was rolled to a thickness of 2 mm with a 7 stand mill of a finish rolling mill and continuously joined and finish-rolled 10 times or more, one or more breaks occurred.

  As shown in FIG. 12, heating is performed until the temperature in the width range of 50 to 100% is equal to or higher than the liquidus temperature of the steel slab in the entire width in each joining surface of the preceding steel slab and the subsequent steel slab in the heating process, In the finish rolling of the subsequent process, the upset amount in the joining process is 1.1 to 7.0 times the sum of the maximum melting depth of the preceding steel slab and the maximum melting depth of the subsequent steel slab. It was found that the joint can be prevented from breaking.

  In the present invention examples (Nos. 45 to 50), good rolling could be continued without separation of the joints after rolling up to a plate thickness of 2 mm.

  On the other hand, in the comparative example (No. 51, 52), the ratio of making the full width heated above the liquidus temperature of the steel slab in the heating step is either the preceding steel slab or the subsequent steel slab, or the preceding steel slab. Since both the subsequent steel slabs were less than 50%, the joint part was separated during the continuous rolling.

  In Comparative Examples (Nos. 53 and 54), the upset amount / (the sum of the maximum melting depths) was less than 1.1 in the joining process, so the joints were separated during the continuous rolling.

  Moreover, in the comparative example (No. 55), buckling occurred during the above-described continuous rolling because the amount of upset / (sum of maximum melting depth) exceeded 7.0 in the joining process.

DESCRIPTION OF SYMBOLS 1 Coil box 2 Crop shear 3 Joining device 4 Leveler 5 Pinch roll 6 Descaling device 7 First stand of finishing mill 8, 9 Clamp device 10 Induction heating device 11 Pressing cylinder 13 Magnetic pole core 14 Coil 15 Power supply 16 Looper 17 Carriage 18 Steel Roller Table Roller 19 Rail 20 Misalignment Prevention Plate S1 Preceding Steel Plate S2 Backing Steel Slab

Claims (3)

The leading edge of the preceding steel slab and the leading edge of the following steel slab are placed in contact with each other in a non-contact manner on the entry side of the finish rolling mill of the continuous hot rolling line, and the tail edge of the preceding steel slab and the following steel slab A method for joining steel slabs in continuous hot rolling, comprising: a heating step for heating the tip, and a joining step for pressing and joining the tail end of the heated preceding steel slab and the tip of the succeeding steel slab In
In the heating step, heating is performed until the temperature in the range of 50 to 100% of the total width of the joining surface at the tail end of the preceding steel slab and the tip of the subsequent steel slab becomes equal to or higher than the liquidus temperature of the steel slab. And
And the amount of upset in the joining step is 1.1 to 7.0 times the sum of the maximum melting depth of the preceding steel slab and the maximum melting depth of the succeeding steel slab. A method of joining steel slabs in continuous hot rolling.   A continuous hot rolling method using the joining method according to claim 1.   A method for producing a hot-rolled steel sheet, wherein the continuous hot rolling method according to claim 2 is used.

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