JP2008213015A - Manufacturing method of cold-rolled steel strip, and continuous treatment line for steel strip - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To surely correct the shape of a steel strip without causing breakage, even in the case of a thin steel strip or in the case when yield strength of the steel strip is increased and elongation (ductility) is reduced due to work-hardening, when correcting the shape of the steel strip such as flatness by using a tension leveller. <P>SOLUTION: The steel strip 1, after cold rolling, is subjected to heat treatment at the temperature of ≥70°C and ≤500°C with a heat treatment facility 10 and then the shape is corrected by the tension leveller 101. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing a cold-rolled steel sheet excellent in shape such as flatness and a continuous processing line for the steel sheet.

  In steel sheets, particularly cold-rolled steel sheets, shapes such as flatness and warpage are extremely important in terms of quality. Here, as shown in FIG. 9, the flatness failure means that the steel plate (hereinafter referred to as including a steel strip) is locally wavy, such as (a) ear extension, (b) belly extension, etc. This means that the degree is above a certain level. In FIG. 9, 1 indicates a steel plate.

  As an apparatus for correcting the shape of such a steel plate, there is a tension leveler as shown in Non-Patent Document 1. The tension leveler is a steel sheet that is stretched by bending and unbending with a roll while applying tension to the steel sheet, causing the flatness of the steel sheet to be elongated in the longitudinal direction of the steel sheet. Refers to the correction device that resolves.

  As shown in FIG. 10, the tension leveler is composed of two rolls (also referred to as “stretching”) roll units 120 and 130, one work roll, and front and rear thereof. A type composed of a straightening roll unit 140 made of a support roll is generally used.

  The tension leveler 101 has a structure in which one work roll of the extension roll units 120 and 130 and two work rolls of the straightening roll unit 140 can move up and down together with a backup roll group (not shown) that holds the work roll. The type is generally.

  In Patent Document 1, a heating roll capable of forming a temperature gradient in the roll axis direction (the width direction of the steel sheet) is provided on the entry side of the tension leveler, and after forming a temperature difference in the width direction of the steel sheet with this heating roll, A tension leveler and a straightening method for straightening a steel plate with an extension roll unit and a straightening roll unit are disclosed.

By the way, as an index indicating the degree of shape failure of a steel plate, what is called steepness is used in the case of ear extension or belly extension. As shown in FIG. 11, when the steel plate 1 shown in FIG. In case,
λ = d ÷ L × 100 (%) (1)
It is expressed.
"Current status and problems of shape correction technology for levelers and skin pass mills" (Japan Society for Technology of Plasticity, 171st Symposium on Plasticity Processing, P61-69) JP 2000-042632 A

  As described above, the importance of shape correction of steel sheets is increasing, and it is particularly important to ensure the quality of flatness. The principle of shape correction with the tension leveler is the cause of poor flatness by making the steel sheet stretched while being bent and unbent by sequentially bending the steel sheet by pressing the stretching roll into contact with the tensioned steel sheet. The distribution in the longitudinal direction of the steel sheet is eliminated in the width direction of the steel sheet. And the curvature of the steel plate which generate | occur | produces by the bending by an extending | stretching roll, and a bending return is eliminated by adjustment of the extrusion position of the roll for curvature correction.

  In recent years, steel plates, particularly cold-rolled steel plates, have come to handle products with extremely thin plate thickness. In addition, the allowable range of flatness is becoming stricter. Here, the product having a very thin plate thickness specifically refers to a product having a thickness of 0.3 mm or less, and is manufactured by performing cold rolling on a hot-rolled mother plate having a thickness of 2 mm.

  The cold-rolled steel sheet having such a thickness has high yield strength and low elongation (ductility) due to work hardening in cold rolling. Work hardening is a phenomenon in which a metal hardens as strain increases, and is attributed to the ease of dislocation movement within metal crystal grains.

  When trying to correct the shape of a cold-rolled steel sheet with such a thin thickness and work-hardened with a tension leveler as disclosed in Non-Patent Document 1, the yield strength is high, so flatness is corrected. A large tension is required to secure the necessary elongation.

  Further, since the effect of bending by the roll becomes smaller as the plate thickness becomes thinner, a larger tension is required. Moreover, since the work-hardened material has reduced elongation (ductility), the risk of breakage increases.

  Patent Document 1 uses a heating roll installed on the tension leveler entrance side to generate a temperature difference in the width direction of the steel sheet, and utilizes the tension difference due to the difference in thermal expansion of the steel sheet to increase the longitudinal direction of the steel sheet with the tension leveler. Although it is intended to control the distribution in the width direction of the steel sheet, in the case of a cold-rolled steel sheet having a very thin thickness, the temperature difference due to cooling or contact with the roll is large, so that even if the tension leveler corrects the tension difference described above. There is a problem that the desired effect cannot be obtained.

  The present invention has been made to solve the problems of the prior art as described above, and an object of the present invention is to provide a method for producing a cold-rolled steel sheet having excellent shapes such as flatness.

  The present invention for achieving the above-mentioned object is characterized in that, firstly, a steel sheet after cold rolling is subjected to a heat treatment at a temperature of 70 ° C. or more and 500 ° C. or less, and then the shape is corrected with a tension leveler. It is the manufacturing method of the cold-rolled steel plate to do.

  The second aspect of the present invention is the method for producing a cold-rolled steel sheet according to the first aspect of the present invention, wherein the heat treatment is performed for 1 second to 100 seconds.

  Furthermore, the present invention thirdly relates to the method for producing a cold-rolled steel sheet according to the first aspect of the present invention, wherein the total rolling reduction in cold rolling is 20% or more and 99.5% or less.

  Fourthly, the present invention is a continuous processing line for steel sheets provided with a heat treatment facility upstream of the tension leveler in a continuous processing line for steel sheets provided with a tension leveler.

  According to the present invention, when correcting the shape such as the flatness of the steel sheet with a tension leveler, when the steel sheet thickness is extremely thin, or when the yield strength of the steel sheet increases due to work hardening and the elongation (ductility) decreases. Even so, the shape can be reliably corrected without breaking the steel plate.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(Embodiment of the present invention)
The principle of straightening the shape with a tension leveler is that steel plates with poor flatness have a distribution in which the elongation in the longitudinal direction changes when viewed in the width direction of the steel plate. This is to give a small elongation strain to a small portion. At that time, as the elongation in the longitudinal direction, it is necessary to give an elongation greater than the maximum and minimum difference when viewed in the width direction of the steel sheet.

  The outline of the tension leveler will be described with reference to FIG. In FIG. 10, bridle rolls 201 and 301 for applying tension to the steel plate 1 are installed before and after the tension leveler 101 (the front indicates the upstream side in the conveying direction of the steel plate 1 and the rear indicates the downstream side). Has been. The tension leveler 101 includes two extending roll units 120 and 130 and a straightening roll unit 140. The extension roll units 120 and 130 are each composed of a pair of upper and lower rolls. By moving the upper and lower positions of the upper roll or the lower roll, the stretch roll units 120 and 130 are in a state where they are just in contact with the steel sheet. Adjust the push depth of the roll in the vertical direction (also called intermesh).

  Since the diameter of the upper and lower rolls is small, the extension roll is normally supported by a plurality of backup rolls, although not shown in FIG. The correction roll unit 140 is for correcting the warp of the steel sheet 1 generated by the bending and unbending in the extension roll units 120 and 130. The straightening roll unit 140 arranges a roll with a relatively small diameter between two rolls with a relatively large diameter, and adjusts the indentation depth (intermesh) of the roll, thereby correcting the warpage of the steel sheet 1. We are trying to correct it.

  On the other hand, a change in material properties in cold rolling will be described by taking a case of SPCC steel (JIS G3141) which is a typical cold rolled steel sheet as an example. 2 and 3 show a tensile test (JIS Z22015 No. 5) of a cold-rolled steel sheet obtained by performing stepwise cold rolling on a pickled SPCC base plate having a thickness of 2 mm, and yield strength and elongation. It is the result of investigating.

  From FIG. 2, the yield strength increases as the total rolling reduction of cold rolling increases. From FIG. 3, the elongation decreases as the total rolling reduction of cold rolling increases. This is due to work hardening of the cold rolled steel sheet. In particular, when the total rolling reduction of cold rolling exceeds 20%, the yield strength is large and the elongation is greatly reduced. Naturally, when the total rolling reduction of the cold rolling is large, the sheet thickness after rolling is thin.

  When cold-rolled steel sheets with such a thin plate thickness, high yield strength, and reduced elongation are to be straightened with a tension leveler, a large tension is required and the elongation (ductility) of the material is small. The risk of breaking increases.

  On the other hand, in order to improve the material of the work-hardened steel sheet, there is a heat treatment method called annealing, and heat treatment using a batch-type annealing furnace or a continuous annealing furnace (also referred to as CAL) has been conventionally performed in the steel sheet manufacturing process. ing.

  Annealing is a heat treatment for heating to a temperature at which the metal structure recrystallizes, and the recrystallization temperature of the steel sheet is as high as 700 ° C. or higher. Therefore, the equipment for annealing becomes large and the equipment cost is high. In addition, a large amount of heat energy is required for annealing, and manufacturing costs are also increased.

  The inventors of the present invention do not require costly heat treatment such as annealing, and a method for easily correcting the shape of a cold-rolled steel sheet having a very thin plate thickness, a high yield strength, and a reduced elongation with a tension leveler. The inventors have intensively studied to arrive at the present invention.

  An experiment was conducted in which a heat treatment in a wide temperature range was performed on a cold-rolled steel sheet cold-rolled (total reduction ratio of 75%) of the SPCC steel having a thickness of 2.0 mm to 0.5 mm. When the yield strength and elongation (ductility) of the cooled cold-rolled steel sheet were measured, the inventors found that the material could be improved even when heat treatment was performed at a temperature lower than the recrystallization temperature.

  Here, an infrared heater was used as the heating device. FIG. 4 shows the relationship between the heat treatment temperature and the yield strength. Here, the heat treatment time was 10 seconds. The yield strength of the cold-rolled steel sheet that has been cold-rolled with a total rolling reduction of 75% is 900 MPa. From FIG. 4, even when heat treatment is performed at a temperature lower than the recrystallization temperature (700 ° C. or higher), the yield strength is reduced. It can be seen that the strength has decreased.

  FIG. 5 shows the relationship between the heat treatment temperature and elongation. The elongation of a cold-rolled steel sheet that has been cold-rolled with a total rolling reduction of 75% is 0.1% or less, but from FIG. 5, the heat treatment is performed at a temperature lower than the recrystallization temperature (700 ° C. or higher). However, it can be seen that the elongation has improved.

  Thus, the heat treatment for improving the material has conventionally required a temperature higher than the recrystallization temperature. However, according to the present invention, even if the heat treatment is performed at a temperature lower than the recrystallization temperature, the material is improved. The reason for this is that, due to the movement and diffusion of metal atoms caused by heating, atomic vacancies introduced during work hardening by cold rolling disappear, or rearrangement of dislocations occurs. Presumed to happen.

  If the yield strength of the cold-rolled steel sheet is reduced and the elongation is improved, the risk of breakage when performing shape correction with a tension leveler is reduced, and a steel sheet with good flatness can be obtained.

  Moreover, the result of having investigated about the influence of the time of heat processing is shown. In the cold-rolled steel sheet that had been cold-rolled with a total rolling reduction of 75%, the material when the heat treatment time was sequentially changed at a heating temperature of 300 ° C. was investigated. FIG. 6 shows the relationship between heat treatment time and elongation. From FIG. 6, it was found that even if the heat treatment time is extremely short, the effect of improving the material is manifested. Here, the case of elongation as a material has been described. However, it has been separately confirmed that the yield strength is reduced even in an extremely short heat treatment time.

  Further, heat treatment experiments were similarly conducted on cold-rolled steel sheets having different total rolling reductions, and changes in materials were investigated. The heat treatment conditions were 300 ° C. and 10 seconds. FIG. 7 shows the relationship between the total rolling reduction and the amount of change in elongation. Here, the amount of change in elongation indicates how much the elongation has improved before and after heat treatment.

  From FIG. 7, it can be seen that when the total rolling reduction is small, the amount of change in elongation is small, and when the total rolling reduction is increased to a certain degree or more, the amount of change in elongation is large and the effect of increasing the elongation is manifested.

  This is presumably because when the total rolling reduction is small, the work hardening is small and the elongation does not decrease much, so the margin for improving the elongation by heat treatment is small.

  In the above description, the case of SPCC steel, which is a general cold-rolled steel sheet, has been described as an example, but it has been experimentally confirmed that similar effects can be obtained for cold-rolled steel sheets of other steel types. .

  In this experiment, an infrared heater was used as the heating device, but the same effect can be obtained by other types of heating devices such as electromagnetic induction heating and heating rolls.

  In the first aspect of the present invention, the steel sheet after cold rolling is subjected to a heat treatment at a temperature of 70 ° C. or more and 500 ° C. or less, and then the shape is corrected with a tension leveler.

  In the second aspect of the present invention, in the first aspect of the present invention, the time for performing the heat treatment is set to 1 second to 100 seconds.

  Furthermore, in the third aspect of the present invention, in the first aspect of the present invention, the total rolling reduction in cold rolling is set to 20% or more and 99.5% or less.

  The first aspect of the present invention will be described in detail. In the first aspect of the present invention, as shown in FIG. 1, a heat treatment facility 10 is provided on the upstream side of the tension leveler 101, and the cold-rolled steel sheet 1 before shape correction is subjected to heat treatment.

  The heat treatment temperature is set to 70 ° C. or higher, as shown in FIGS. 2 and 3, when the heat treatment temperature is 70 ° C. or higher, the material improvement effect of cold-rolled steel sheet (reduction in yield strength, improvement in elongation) is achieved. This is because the risk of appearing and breaking by shape correction with a tension leveler decreases.

  Moreover, the upper limit of the heat treatment temperature is set to 500 ° C. If a heat treatment at a temperature higher than 500 ° C. is to be performed, a large-scale heat treatment facility is required, which increases the equipment cost and increases the energy required for the heat treatment. This is because the manufacturing cost becomes high.

  Here, once the heat-treated cold-rolled steel sheet is straightened by the tension leveler 101, the temperature may be lowered to room temperature, so the heat treatment equipment 10 is located at any position upstream of the tension leveler 101. It does not matter. In addition, the heat treatment may be performed in advance in a separate process in a different line from where the tension leveler is installed.

  Next, the second aspect of the present invention will be described in detail. In the second aspect of the present invention, in the first aspect of the present invention, the time for performing the heat treatment is set to 1 second to 100 seconds.

  As shown in FIG. 6, the time for performing the heat treatment is 1 second or longer. As shown in FIG. 6, even when the heat treatment time is extremely short, it is confirmed that the material improvement effect is exhibited. This is because the risk of breakage due to shape correction by the tension leveler 101 is reduced.

  The upper limit of the heat treatment time is set to 100 seconds because the material improvement effect is saturated even if the heat treatment time is long. From the viewpoint of saving thermal energy, the transfer speed of the steel sheet 1 and the reality of the size of the heat treatment equipment 10 By deciding such an upper limit.

  Next, the third aspect of the present invention will be described in detail. In the second aspect of the present invention, in the first aspect of the present invention, the total rolling reduction in the cold rolling is set to 20% or more and 99.5% or less.

  The total rolling reduction in cold rolling is set to 20% or more, as shown in FIG. 7, when the total rolling reduction is less than 20%, work hardening in cold rolling is originally small, so the material improvement effect by heat treatment This is because the material improvement effect by the heat treatment is not fully exhibited until 20% or more.

  The upper limit is 99.5%, for example, when the total rolling reduction is 99.5% with respect to a hot-rolled mother plate having a thickness of 2.0 mm, the plate thickness after cold rolling is 0.01 mm. This is because the thickness of the steel sheet manufactured industrially is out of the practical range.

  Although it is the 4th present invention, the above operation of the present invention is realizable in the line provided with heat treatment equipment 10 in the upper stream side of tension leveler 101 as shown in FIG. It is preferable from the viewpoint of securing production efficiency that such a line is provided with a tension leveler in a continuous processing line for steel plates.

  As explained above, when correcting the shape such as flatness of a steel sheet with a tension leveler, even if the sheet thickness is extremely thin, or even if the steel sheet has increased yield strength and reduced elongation (ductility) due to work hardening The shape can be surely corrected without breaking the steel plate.

(Example 1)
The present invention will be described below with reference to an embodiment in which the heat treatment equipment 10 is installed on the entrance side of the tension leveler line as shown in FIG.

  In FIG. 8, 401 is a steel sheet wound in a coil shape being unwound, 201 and 301 are bridle rolls for applying tension to the steel sheet, and 402 is a steel sheet wound in a coil shape being wound. It is. The tension leveler 101 is installed in a section where the steel sheet is tensioned by the bridle rolls 201 and 301.

  A contact type plate speed meter 302 is installed on the input / output side of the tension leveler 101. As shown in FIG. 8, the tension leveler 101 according to the first embodiment has a structure in which the support roll 143 in the last stage can be moved up and down together with the work roll 142 in the straightening roll unit 140. The diameter of the work roll of the extension roll units 120 and 130 is 40 mm, the diameter of the work roll 142 of the straightening roll unit 140 is 40 mm, and the diameter of the support roll 143 is 150 mm. Further, the heat treatment equipment 10 is installed on the upstream side of the bridle roll 201 on the entry side. This heat treatment facility 10 uses an infrared heater as a heating device.

  The test materials for the experiment are SPCC steel and SPCD steel as shown in Table 1. In either case, the base plate that has been subjected to pickling after hot rolling is cold-rolled (cold-rolled) to a predetermined plate thickness, and the tension leveler 101 corrects the shape. A 6-stand tandem rolling mill was used for cold rolling.

  Table 1 shows invention examples and comparative examples.

  In Invention Example 1, a tension leveler 101 is used for a cold-rolled steel sheet that has been subjected to cold rolling with a total reduction ratio of 90.0% up to a thickness of 0.20 mm on a SPCC mother board having a thickness of 2.0 mm before cold rolling. Before performing the shape correction, the heat treatment equipment 10 performed heat treatment at 300 ° C. for 10 seconds, and then performed the shape correction with the tension leveler 101. The target elongation rate required for shape correction is set to 0.5% based on the results of the investigation in advance, the intermesh of the elongation roll units 120 and 130 is set to the upper limit of 25 mm, and the intermesh of the correction roll unit 140 is The leveler tension by the tension leveler 101 was adjusted appropriately so as not to warp, and was 300 MPa when controlled by adjusting the speed of the bridle roll so as to obtain the target elongation rate.

  Hereinafter, the settings of the intermesh and the leveler tension of each roll of the tension leveler 101 are set in the same manner as in the first invention example.

  In Invention Example 2, a tension leveler 101 was used for a cold-rolled steel sheet that had been subjected to cold rolling with a total reduction ratio of 92.5% up to a thickness of 0.15 mm on a SPCC mother board having a thickness of 2.0 mm before cold-rolling. Before performing the shape correction, the heat treatment equipment 10 performed heat treatment at 300 ° C. for 10 seconds, and then performed the shape correction with the tension leveler 101. The target elongation was set at 0.5%, and the leveler tension was controlled at 350 MPa.

  In Invention Example 3, a tension leveler 101 was used for a cold-rolled steel sheet that had been subjected to cold rolling with a total reduction ratio of 88.6% up to a plate thickness of 0.25 mm on a SPCD mother board having a thickness of 2.2 mm before cold rolling. Before performing the shape correction, the heat treatment equipment 10 performed the heat treatment at 450 ° C. for 5 seconds, and then performed the shape correction with the tension leveler 101. The target elongation was set at 0.6%, and the leveler tension was controlled at 280 MPa.

  In Invention Example 4, a tension leveler 101 was used for a cold-rolled steel sheet that had been subjected to cold rolling with a total reduction ratio of 91.8% up to a thickness of 0.18 mm on a SPCD mother board having a thickness of 2.2 mm before cold rolling. Before performing the shape correction, the heat treatment facility 10 was subjected to heat treatment at 200 ° C. for 30 seconds, and then the shape level was corrected by the tension leveler 101. The target elongation was set at 0.6%, and the leveler tension was controlled at 320 MPa.

  In Comparative Example 1, a tension leveler 101 was used for a cold-rolled steel sheet that had been subjected to cold rolling with a total reduction ratio of 90.0% up to a thickness of 0.20 mm on a SPCC base plate having a thickness of 2.0 mm before cold-rolling. The shape was corrected by the tension leveler 101 without performing heat treatment before the shape correction. The target elongation was set to 0.6%, and the leveler tension was controlled to 500 MPa.

  In Comparative Example 2, a tension leveler 101 was used for a cold-rolled steel sheet that had been subjected to cold rolling with a total reduction ratio of 91.8% up to a thickness of 0.18 mm on a SPCD mother board having a thickness of 2.2 mm before cold rolling. The shape was corrected by the tension leveler 101 without performing heat treatment before the shape correction. When the leveler tension was set to 300 MPa, which has a very low risk of breaking, the elongation was 0.2%.

  About the said invention example and a comparative example, the sample is extract | collected from the steel plate 402 wound in the coil shape after passing the tension leveler 101, the flatness of a steel plate is investigated, and the presence or absence of generation | occurrence | production of the fracture | rupture at the time of passing plate Was evaluated. When the average steepness was within 0.5%, the shape when the plate-passing experiment was performed 10 times under each condition was evaluated as ◯ (good), and when it was larger than 0.5%, it was rated as x (bad). Regarding the occurrence of breakage, in 10 plate-passing experiments, if no breakage occurred even once (good), if breakage occurred even once, x (bad).

  As a comprehensive evaluation, if both the flatness and the occurrence of breakage were ◯ (good), it was evaluated as ◯ (good), and when either one was x (defective), it was evaluated as x (defective).

  In Table 1, the flatness of each of Invention Examples 1 to 4 was good (good). This is because a necessary elongation rate is given under any condition, and sufficient shape correction is achieved. Regarding the occurrence of breakage, Invention Examples 1 to 4 were ○ (good) because a tension leveler threading plate with a low tension was possible.

  On the other hand, in Comparative Example 1, the flatness was ◯ (good), but the occurrence of breakage was x (bad). This is because a high leveler tension is required to secure the target elongation rate.

  In Comparative Example 2, when the plate passing tension was set low in order to prevent the occurrence of breakage, the occurrence of breakage was ○ (good), but a sufficient elongation rate for shape correction could not be secured, so the flatness Was x (bad).

  Therefore, as an overall evaluation, all of the inventive examples were ◯ (good), but the comparative examples were x (bad).

  From the above results, according to the present invention, when correcting the shape such as the flatness of the steel plate with a tension leveler, the yield strength increased due to work hardening and the elongation (ductility) decreased when the plate thickness was extremely thin. Even in this case, the shape can be reliably corrected without breaking the steel plate.

Diagram showing an example of an embodiment of the present invention Diagram showing the operation of the present invention Diagram showing the operation of the present invention Diagram showing the operation of the present invention Diagram showing the operation of the present invention Diagram showing the operation of the present invention Diagram showing the operation of the present invention Diagram showing an example of a line to which the present invention should be applied Diagram for explaining flatness of steel sheet Diagram for explaining tension leveler Diagram showing the definition of steepness

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Steel plate 10 Heat treatment equipment 101 Tension leveler 120,130 Elongation roll unit 140 Straightening roll unit 142 Work roll 143 Support roll 201,301 Bridle roll 302 Plate speedometer 401,402 Steel plate wound in coil shape

Claims (4)

A method for producing a cold-rolled steel sheet, comprising subjecting a steel sheet after cold rolling to a heat treatment at a temperature of 70 ° C to 500 ° C, and then correcting the shape with a tension leveler. 2. The method of manufacturing a cold-rolled steel sheet according to claim 1, wherein the time for performing the heat treatment is 1 second or more and 100 seconds or less. The method for producing a cold-rolled steel sheet according to claim 1, wherein the total rolling reduction in cold rolling is 20% or more and 99.5% or less. A continuous processing line for steel sheets provided with a tension leveler, comprising a heat treatment facility upstream of the tension leveler.

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