JP6070616B2 - Manufacturing method of hot-rolled steel sheet - Google Patents

Description

  The present invention relates to a method for producing a hot-rolled steel sheet, and more particularly to a method for producing a hot-rolled steel sheet that is suitable for use in an automobile body or the like and that suppresses the occurrence of warpage during cutting.

  In recent years, from the viewpoint of global environmental conservation, the weight reduction of automobile bodies has been promoted, and steel sheets having high strength that can be thinned have been actively used as steel sheets used in automobile bodies. . In particular, hot-rolled steel sheets having a tensile strength of 540 MPa or more are used for structural parts of automobiles.

  Hot-rolled steel sheets used for automobile bodies and the like are subjected to a hot rolling process in which a steel slab is hot-rolled into a hot-rolled steel sheet in a hot rolling line, and then wound into a coil shape. In a shape correction processing line having a skin pass mill (also referred to as a temper rolling mill), a shape correction process is performed in which the shape correction processing of the steel sheet is performed while rewinding the coiled steel sheet, and the coil is wound again. Shipped. The shipped hot-rolled steel sheet is cut into a desired size and shape by punching or the like in a production line for an automobile body, and is subjected to press working or the like to have a desired shape. Such an automobile body production line is being automated, and severe flatness is required for the cut steel sheet.

  On the other hand, with the increase in strength of hot-rolled steel sheets, warpage that occurs when the hot-rolled steel sheets are cut increases, and it has become difficult to ensure good flatness. More recently, as a technique for placing steel sheets used in automobile bodies in the right place according to the required characteristics, for example, by laser welding, multiple steel sheets with different materials and thicknesses can be used to reduce the weight of automobile bodies. A so-called tailored blank technique is studied in which a single blank plate is joined before press molding. When a tailored blank material is produced by laser welding, strict flatness is particularly required for steel plates to be joined.

  One of the causes of warpage of the steel sheet after being cut into a predetermined shape to deteriorate the flatness is the residual stress of the steel sheet. Focusing on the residual stress of such a steel sheet, for example, a technique described in Patent Document 1 has been proposed as a technique for suppressing warpage or the like when cutting the steel sheet and improving the flatness of the steel sheet. .

  The technique described in Patent Document 1 relates to a high-strength steel plate suitable for automobile parts such as a truck frame, and by defining the residual stress inside the steel plate, its distribution, and the optimum conditions of the steel plate manufacturing method. This is a technique for obtaining a high-strength steel sheet having a good shape after cutting. In Patent Document 1, as a method for producing a high-strength hot-rolled steel sheet having a good shape after cutting, steel having a predetermined component composition is finish-rolled, cooled at a predetermined cooling rate, and wound into a coil shape. A method is described in which, after cooling to below ℃, it is subjected to a cutting line having a roll leveler and corrected under predetermined conditions. Patent Document 1 discloses that an average cooling rate difference in the plate width direction of a steel strip (hereinafter also referred to as a steel plate) is set to 5 ° C./min or less on a run-out table between hot rolling and winding. When the steel sheet is wound into a coil shape and then cooled to 100 ° C. or less, the average cooling rate difference in the width direction of the steel sheet is set to 5 ° C./min or less. After temper rolling at .5% or less, or after performing tension correction with a tension leveler or the like at an elongation of 1.5% or less, it is necessary to coil the steel sheet and then use it for the cutting line. It is described that it can be added accordingly.

JP 2000-212688 A

  However, even with the technique described in Patent Document 1, sufficient correction may not be possible depending on the shape of the steel sheet to be cut and the ability of the roll leveler. Generally, in order to improve the shape of the steel sheet before cutting, shape correction by a skin pass mill or a tension leveler is performed. Even in the technique described in Patent Document 1, the shape defect of the steel sheet is improved by performing temper rolling at a rolling reduction of 1.5% or less after the steel sheet is wound into a coil shape and before the cutting process. It is stated that the removal of residual stress by the roll leveler is improved, and that the residual stress can be remarkably reduced in the straightening / cutting process with the roll leveler by performing tension correction with a tension leveler at an elongation of 1.5% or less. Has been.

  However, in the production of hot-rolled steel sheets having a tensile strength of 540 MPa or more, the steel sheets are coiled and then subjected to temper rolling and straightening with a tension leveler before the cutting process, and the steel sheets before cutting. Even if the shape was good, there were cases where sufficient flatness could not be obtained after cutting. The present invention solves the above-mentioned problems of the prior art and provides a method for producing a hot-rolled steel sheet having a tensile strength of 540 MPa or more and excellent in flatness after cutting. In addition, the hot-rolled steel plate which this invention makes object is plate | board thickness: About 1.2-6.0 mm, Sheet | seat width: About 500-2300 mm, The tensile strength is 540 Mpa or more.

  The inventors of the present invention have a shape correction treatment using a skin pass mill or a tension leveler in order to suppress the warpage of the hot-rolled steel sheet after cutting, in particular, for the hot-rolled steel sheet manufactured in the hot rolling process. Various process conditions were studied. In the shape correction treatment of hot-rolled steel sheets, the hot-rolled steel sheets wound in a coil shape are rewound and processed with a skin pass mill or tension leveler under the condition of applying tension to give the steel sheets elongation and shape correction treatment. And take up again in a coil. Here, as operating conditions of the skin pass mill and the tension leveler, as shown in Patent Document 1, hot rolling that is a material to be treated, such as a reduction rate of temper rolling (also referred to as a skin pass) and an elongation rate of the tension leveler. The deformation amount of the steel plate was set to be a predetermined value.

  However, as a result of detailed investigations by the inventors, when a shape correction treatment is performed with a constant deformation amount applied to the steel plate by a skin pass mill or a tension leveler, and the shape of the steel plate before cutting is improved, hot rolling after cutting is performed. There was a case where a large warp occurred in the steel plate. As a result of repeated investigations by the inventors paying attention to such a phenomenon, warping that occurs after cutting of such a steel sheet is caused by a reduction rate and elongation rate at which the deformation amount of the hot-rolled steel sheet becomes a constant amount. It has been found that the residual stress accumulated in the hot-rolled steel sheet increases due to the application to the rolled steel sheet. As a result of further investigation, the temperature difference in the sheet width direction of the coiling temperature is set to 50 ° C. or less, and the cooling distortion in the sheet width direction of the hot rolled steel sheet is suppressed, and then given to the hot rolled steel sheet in the shape correction treatment. It has been found that by suppressing the fluctuation range of the tension, the flatness can be improved by suppressing the warpage of the steel sheet after cutting. This invention is completed based on such knowledge, The summary structure is as follows.

  (1) By mass%, C: 0.01 to 0.20%, Si: 2.0% or less (including 0%), Mn: 0.2 to 2.0%, P: 0.05% or less , S: 0.010% or less, Al: 0.1% or less (including 0%) steel slab is hot-rolled into a hot-rolled steel sheet, and then the coiling temperature in the sheet width direction The difference is set to 50 ° C. or less, and then a predetermined tension that gives a target elongation rate within a range of 0.1 to 3.0% is set on the hot-rolled steel sheet, and the fluctuation range of the tension is set to the predetermined range. The shape correction treatment is performed while applying tension as a value of -20.0% to + 20.0% of the thickness of the plate, thickness: 1.2-6.0 mm, plate width: 500-2300 mm, tensile strength: The manufacturing method of the hot-rolled steel plate which is 540 MPa or more.

  According to the present invention, the occurrence of sheet warpage during cutting can be suppressed, and a high-strength hot-rolled steel sheet having excellent flatness can be obtained, so that production efficiency in an automobile body production line or the like is improved. Can do.

It is a schematic diagram of a shape processing line. It is a schematic diagram which shows the measurement position of the board curvature of a hot-rolled steel plate (test piece).

  Hereinafter, an embodiment of the present invention will be described. In addition, unless otherwise indicated, the "%" display regarding a component composition shall mean "mass%".

  First, the component composition of the steel slab will be described.

C: 0.01 to 0.20%
C is an element effective for increasing the tensile strength of the hot-rolled steel sheet, and in order to achieve a tensile strength of 540 MPa or more, the C content is 0.01% or more. More preferably, it is 0.03% or more. On the other hand, when the amount of C exceeds 0.20%, the hardenability is greatly increased, and in the cooling after hot rolling described later, the residual stress in the hot-rolled steel sheet becomes large, and the shape of the hot-rolled steel sheet tends to deteriorate. . Therefore, the C amount is 0.20% or less. More preferably, it is 0.18% or less.

Si: 2.0% or less (including 0%)
If the Si content exceeds 2.0%, the surface properties are significantly deteriorated due to the generation of oxide scale, so the Si content is 2.0% or less. More preferably, it is 1.5% or less. In the present invention, the Si amount may be 0%, but since Si has a solid solution strengthening ability and is an element effective in increasing the tensile strength of the hot-rolled steel sheet, It may be added to increase the strength.

Mn: 0.2 to 2.0%
Mn is an element that increases the strength of the hot-rolled steel sheet, and is a useful element that suppresses surface cracking caused by hot brittleness due to S. In order to obtain these effects, the amount of Mn is 0.2% or more. More preferably, it is 0.5% or more. On the other hand, if the amount of Mn exceeds 2.0%, the hardenability increases, and the residual stress tends to increase due to variations in cooling conditions after hot rolling. Therefore, the Mn content is 2.0% or less.

P: 0.05% or less P is an element that lowers the workability and corrosion resistance of the hot-rolled steel sheet. If the amount of P exceeds 0.05%, the effect becomes significant, so the amount of P is set to 0.05% or less. More preferably, the amount of P is 0.03% or less.

S: 0.010% or less S is an element that decreases the ductility and toughness of steel. If the amount of S exceeds 0.010%, the effect becomes significant, so the amount of S is made 0.010% or less. More preferably, the amount of S is 0.005% or less.

Al: 0.1% or less (including 0%)
Al is an element that acts as a deoxidizer and improves the cleanliness of steel. In order to obtain such an effect, the Al content is preferably 0.001% or more. If the Al content exceeds 0.1%, the effect of improving the cleanliness is saturated, and problems such as an increase in manufacturing cost and an increase in the occurrence tendency of surface defects are likely to occur. For this reason, the amount of Al is made 0.1% or less. The Al amount may be 0%.

  In addition to the above components, the steel slab used in the present invention preferably uses carbide forming elements such as Nb, Ti, V, and Mo in order to increase the strength of the hot-rolled steel sheet using precipitation strengthening. . These elements contribute to an increase in the tensile strength of the hot-rolled steel sheet by forming fine carbides in the hot-rolled steel sheet. In order to obtain this effect, Nb: 0.01 to 0.05%, Ti: 0.01 to 0.15%, V: 0.1 to 0.3%, Mo: 0.1 to 0.3% It is preferable to add one or more.

  The balance other than those described above is Fe and inevitable impurities. However, in order to improve desired characteristics of the hot-rolled steel sheet, elements other than those described above can be added as appropriate. For example, in order to increase the tensile strength of hot-rolled steel sheets, one or more of Cr: 0.1 to 1.5%, B: 0.001 to 0.010%, etc., and the form of non-metallic inclusions are controlled. In order to improve the workability of the hot-rolled steel sheet, Ca: 0.001 to 0.010% can be appropriately added.

  In the present invention, the steel slab having the above-described component composition is hot-rolled to form a hot-rolled steel sheet, and then cooled, and the coil is wound at a temperature difference in the sheet width direction of 50 ° C. or less. A shape correction process is performed while setting a predetermined tension that gives a target elongation rate within a range of 3.0%, and applying a tension with the fluctuation range of the tension being −20.0% to + 20.0% of the predetermined value. Apply.

  The steel slab used for hot rolling may be manufactured by a conventionally known method such as a continuous casting method. The steel slab after casting is once cooled or charged into a heating furnace as a hot piece and reheated to a desired temperature (slab heating temperature), followed by hot rolling consisting of rough rolling and finish rolling, A hot-rolled steel sheet is used. Here, when the slab heating temperature is higher than 1250 ° C., the amount of scale generated on the surface layer of the steel slab tends to be remarkably increased, and the productivity is lowered. Note that hot rolling may be performed by a process such as continuous casting-direct feed rolling in which hot rolling is performed directly after casting the steel slab.

  The conditions for hot rolling need not be particularly limited, and may be performed according to a conventional method. In addition, it is preferable that the finish rolling finish temperature of hot rolling shall be 1000 degrees C or less from a viewpoint of making the uniformity of the steel structure of the hot-rolled steel plate obtained favorable. If the finish rolling end temperature is too low, the rolling load increases and the load on the rolling equipment increases. Therefore, the finish rolling end temperature is preferably 800 ° C. or higher.

  The hot-rolled steel sheet after hot rolling is cooled so that the temperature difference in the sheet width direction of the coiling temperature is 50 ° C. or less, and then wound in a coil shape.

  In order to suppress the occurrence of cooling strain, to improve the shape of the hot-rolled steel sheet used for the shape correction treatment described later, and to reduce the residual stress, the coiling temperature is set to a temperature difference of 50 in the sheet width direction of the hot-rolled steel sheet. Must be below ℃ That is, the hot-rolled steel sheet that has been rolled up after being hot-rolled is cooled on the run-out table. When cooling with this run-out table, the temperature difference in the plate width direction is reduced to reduce the cooling distortion due to temperature unevenness in the plate width direction. Specifically, the temperature difference in the width direction of the hot-rolled steel sheet on the run-out table is reduced, and the temperature at the time of winding after cooling, that is, the temperature difference in the sheet width direction of the coiling temperature is set to 50 ° C. or less. The cooling distortion in the sheet width direction of the rolled steel sheet is suppressed, and the shape of the hot-rolled steel sheet used for the shape correction treatment is improved. Moreover, by setting the temperature difference in the sheet width direction of the coiling temperature to 50 ° C. or less, it is possible to reduce the residual stress of the hot-rolled steel sheet used for the shape correction treatment, and to suppress the warp of the hot-rolled steel sheet after cutting. For this reason, the temperature difference of the plate | board width direction of coiling temperature shall be 50 degrees C or less. More preferably, it is 30 degrees C or less. In order to suppress temperature unevenness in the sheet width direction during cooling at the run-out table, the edge of the hot-rolled steel sheet is masked by masking the end part (also referred to as edge part) of the hot-rolled steel sheet that is easily overcooled. It is preferable to prevent overcooling of the part.

  The hot-rolled steel sheet manufactured as described above is rewound in a coil-shaped hot-rolled steel sheet in a shape correction processing line having a tension leveler and a skin pass mill, and tension is applied to the rewound hot-rolled steel sheet. While being applied, a shape correction process is performed in which a shape correction processing device such as a skin pass mill or a tension leveler is used to apply a distortion to correct the shape. Here, in the skin pass mill, the steel plate is rolled to improve the shape of the steel plate, and in the tension leveler, the shape of the steel plate is improved by subjecting the steel plate to which the tension is applied to bending and bending back by a tension leveler roll. Here, the amount of strain applied in the shape correction processing step is appropriately determined according to the strength and shape of the steel sheet. Specifically, the amount of elongation (elongation rate) of the steel sheet in a shape correction processing device such as a skin pass mill or tension leveler is determined by the strength, thickness, shape, etc. of the steel sheet used for the shape correction processing, and the shape correction processing Is given.

  In FIG. 1, the schematic diagram of the shape correction line which has the skin pass mill 3 and the tension leveler 4 is shown. In the shape correction line of FIG. 1, the hot rolled steel sheet S wound up in a coil shape is rewound with a payoff reel 1, and tension is applied with bridle rolls 2, 5, and a reduction is applied with a skin pass mill 3. After being bent and bent back by the leveler 4 and subjected to shape correction processing, it is wound up in a coil shape by the tension reel 6. Although FIG. 1 shows a shape correction line having a skin pass mill and a tension leveler, a shape correction line having only the skin pass mill 3 or only the tension leveler 4 may be used.

  The target elongation given in the shape correction processing step is appropriately determined according to the conditions of the steel plate such as the strength, thickness, and shape of the steel plate before the shape correction processing by experiments or the like. Here, the elongation is adjusted to a predetermined target elongation by adjusting the reduction amount of the skin pass mill, the amount of indentation by the tension leveler (also referred to as intermesh) and the tension applied to the hot-rolled steel sheet S by the bridle rolls 2 and 5. It can be. That is, before the shape correction treatment is performed on the hot-rolled steel sheet, the condition of the skin pass mill 3 and the condition of the tension leveler 4 and the initial predetermined tension applied by the bridle rolls 2 and 5 for giving the determined target elongation rate. Is determined.

  A hot-rolled steel sheet has variations in material properties and shapes even within one hot-rolled steel sheet due to variations in hot rolling conditions and other factors. Conventionally, in order to give the target elongation rate to the hot-rolled steel sheet, the amount of reduction by the skin pass mill, that is, the load of the skin pass mill, the bender is set to a predetermined value, and the amount of pushing by the tension leveler is set to a predetermined value. Above, the tension | tensile_strength given to a hot-rolled steel plate was adjusted so that elongation may become target elongation.

  On the other hand, in the present invention, the predetermined tension F set to give the target elongation rate is such that the fluctuation range of the predetermined tension F is within a range of −20.0% to + 20.0% of the tension F. In this manner, the shape correction process is performed. In the present invention, as described above, after hot rolling, cooling is performed so that the temperature difference in the sheet width direction of the coiling temperature is 50 ° C. or less, and the generation of cooling strain in the hot-rolled steel sheet is suppressed. Thus, the warp after cutting the hot-rolled steel sheet can be suppressed by setting the fluctuation range of the tension F in the shape correction treatment within the above range. If the fluctuation range of the tension F is less than −20.0% of F or exceeds + 20.0% of F, it becomes difficult to suppress warpage of the hot-rolled steel sheet after cutting. As described above, the reason why the flatness of the hot-rolled steel sheet after cutting can be improved by regulating the fluctuation range of the tension in the straightening process is that the residual stress in the hot-rolled steel sheet after the straightening process is compared with the conventional one. It is thought that it can be made smaller. In order to make the fluctuation range of the tension F within the range of -20.0% to + 20.0% of the tension F, the tension may be constant with the speed before and after the shape correction equipment being constant. More preferably, the shape correction processing is performed so that the fluctuation range of the tension F falls within the range of -15.0% to + 15.0% of F.

  Moreover, in this invention, the target elongation rate in a shape correction process shall be 0.1-3.0%. If the target elongation is less than 0.1%, it is difficult to perform sufficient shape correction treatment. On the other hand, if the target elongation exceeds 3.0%, it becomes difficult to provide the elongation due to equipment capacity. It is. In addition, Preferably, target elongation is 0.1 to 2.0%.

  In the present invention, in the shape correction process, the fluctuation range of the tension is specified and the change in the elongation rate is allowed. If the elongation percentage of the rolled steel sheet is within the range of 0.1 to 3.0%, the desired purpose can be achieved, and there is no problem. Further, the elongation rate during the shape correction treatment can be measured by the speed difference before and after the shape correction equipment (difference in the passing speed of the hot-rolled steel sheet).

  In addition, what is necessary is just to set the target elongation rate initially set in the shape correction process suitably according to materials and shapes, such as the intensity | strength of the hot-rolled steel plate which performs shape correction. In addition, the conditions of skin pass mill, tension leveler, and tension between bridle rolls to give this elongation rate are the same as the hot-rolled steel sheet that has the same material and shape as the hot-rolled steel sheet that performs the shape-correcting process. It can be obtained by an experiment using a shape correction processing line.

  In addition, as above-mentioned as a hot-rolled steel plate which performs a shape correction process by this invention, tensile strength is 540 Mpa or more, Moreover, board thickness: 1.2-6.0 mm, board width: About 500-2300 mm. .

  A steel slab having the component composition shown in Table 1 was heated to 1200 ° C. to form a hot-rolled steel sheet by hot rolling at a finish rolling temperature of 900 ° C., cooled on a runout table, and wound up. Table 2 shows the plate width, plate thickness, coiling temperature, and temperature difference in the plate width direction of the hot rolled steel sheet.

Next, for the obtained hot-rolled steel sheet, the target elongation for one coil (steel sheet length: 500 m to 2000 m) of each hot-rolled steel sheet is shown in Table 2 using a shape correction processing apparatus having a skin pass mill and a tension leveler. As the target elongation shown in FIG. At this time, the initially set tension (set tension) for obtaining this target elongation is expressed as unit tension (tension per unit cross-sectional area of the hot-rolled steel sheet) and is shown in Table 2. In the shape correction process, the variation in unit tension in the shape correction process was set within a range of ± 20.0% of the set tension (invention example). For comparison, the shape correction processing was also performed in the case where the variation in tension was allowed (comparative example) so that the elongation rate in the shape correction processing substantially coincided with the target value as in the past. Table 2 shows the maximum value of the unit tension, the minimum value of the unit tension, and the maximum value and the minimum value of the tension fluctuation with respect to the set tension when the shape correction processing is performed. The maximum value and the minimum value of the tension fluctuation are obtained as follows.
Maximum value of tension fluctuation (%) = [(Maximum unit tension) − (Set tension)] / (Set tension) × 100
Minimum value of tension fluctuation (%) = [(Minimum value of unit tension) − (Set tension)] / (Set tension) × 100
Next, the hot-rolled steel sheet after the shape correction treatment was cut into 300 mm × 650 mm (length in the rolling direction × length in the sheet width direction) from the sheet width end side, and a test piece was collected. Ten test pieces obtained in this way were examined for warpage. In the plate warpage measurement, a steel plate (test piece) cut to the above size was placed on a surface plate, and the length of the gap between the surface plate and the steel plate was measured as the amount of warpage. In addition, as shown in FIG. 2, the amount of warpage is measured for a total of six locations at both ends in the rolling direction and at the center in the rolling direction (ac to df) at both ends in the plate width direction of each test piece. Asked. ○ (pass) for hot-rolled steel sheets with all warpage amounts (10 for each 6 points: 60 in total: 60 points) of 5 mm or less, and hot-rolled steel sheets for which warpage amount exceeded 5 mm at one location × ( ). The results are shown in Table 2.

  In addition, a sample was taken from the obtained hot-rolled steel sheet, and in accordance with JISZ2241, a tensile test was conducted using a No. 5 test in which the direction perpendicular to the rolling direction (C direction) was the tensile direction, and the tensile strength ( TS) was measured. The results are shown in Table 2.

  From Table 2, it can be seen that the inventive example manufactured by the manufacturing method of the present invention has a smaller amount of warpage than the comparative example, and the occurrence of plate warpage during cutting is suppressed.

1 Payoff reel 2 Bridle roll 3 Skin pass mill 4 Tension leveler 5 Bridle roll 6 Tension reel S Steel plate 10 Test piece (steel plate)
11 Surface plate

Claims (1)

In mass%, C: 0.01 to 0.20%, Si: 2.0% or less (including 0%), Mn: 0.2 to 2.0%, P: 0.05% or less, S: A steel slab containing 0.010% or less and Al: 0.1% or less (including 0%) is hot-rolled into a hot-rolled steel sheet, and then the temperature difference in the sheet width direction of the coiling temperature is 50. ℃ winding as follows, then the hot-rolled steel sheet, by setting the predetermined tension applied to target growth rate in the range 0.1 to 3.0% varying Dohaba: the value of the predetermined tension The plate thickness is 1.2 to 6.0 mm, the plate width is 500 to 2300 mm, and the tensile strength is that the shape correction treatment is performed while applying tension within the range of −20.0% to + 20.0% . : The manufacturing method of the hot-rolled steel plate which is 540 Mpa or more.

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