JP2011045922A - Method and equipment for temper-rolling steel strip - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for temper-rolling a steel strip, by which a prescribed elongation percentage is secured and shape straightening is made possible within the range of rolling load which is coped with a conventional rolling mill even about high-tension steel which is ≥1,470 MPa in the tensile strength. <P>SOLUTION: By performing the temper-rolling of a steel strip 4 by using a work roll 1, which is ≤300 mm in the diameter and 3.0-10.0 μm in the surface arithmetic mean roughness, the shape of the steel strip is straightened without requiring large-scale equipment and complex control even when it is high tension steel of ≥1,470 MPa in the tensile strength. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a temper rolling method for high-tensile cold-rolled steel sheets (high tensile steel), and more particularly to a temper rolling method for high-tensile steel sheets having a very high tensile strength, called a super high tensile strength, having a tensile strength of 1470 MPa or higher. The steel strip in the present invention includes both the case of a steel plate wound in a coil shape and a cut plate-like steel single plate.

  Most steel sheets used for automobile bodies and home appliances are cold-rolled steel sheets and steel sheets with surface treatments such as plating. This type of steel sheet is manufactured by casting a steel slab, hot rolling the steel slab, and then further cold-rolling to make it thinner.

  The temper rolling is performed by, for example, subjecting the steel strip to a light reduction with a reduction rate of 1% or less by a temper rolling mill after the cold rolling. By applying this temper rolling, the steel strip is uniformly stretched, the shape thereof is corrected, a predetermined flatness is obtained, mechanical properties such as yield point elongation, tensile strength, elongation, and the steel strip Properties such as surface roughness are also improved.

  In recent years, the demand for high-tensile steel (high-tensile steel) has increased with the increase in added value of steel strips, mainly for the purpose of reducing the weight of automobile bodies. In particular, its tensile strength exceeds 980 MPa class and 1180 MPa class. There is a demand for a steel plate having a range of 1470 MPa class or higher called super high strength steel (ultra high tensile steel). When a steel strip made of such high-strength steel is temper-rolled by a temper rolling mill, the steel strip is hard, and therefore a high rolling load is required to impart the necessary elongation to the steel strip. In particular, it has become difficult to give the necessary elongation to high-tensile steel having a thickness of about 0.6 to 2.0 mm, which is often used as a steel plate for automobiles.

  In particular, steel sheets manufactured by continuous annealing accompanied by quenching and tempering among high-strength steels are prone to deformation due to thermal stress during the quenching process and phase transformation of the steel sheet structure, and form defects are likely to occur. Deforms into a wave shape in the direction. Such a defective shape of the steel sheet is difficult to eliminate even if the steel sheet surface is flattened by cold rolling before annealing. Therefore, it is necessary to correct the shape of the annealed steel sheet by temper rolling, but in the case of a high-tensile steel sheet having a tensile strength of 980 MPa or more, since the deformation resistance is high to give the elongation necessary for shape correction, A very high rolling load is required.

  The higher the strength steel that requires shape correction, the greater the rolling load, and it may be difficult to cope with existing temper rolling mills. Therefore, it is the actual situation to cope by correcting the shape in the subsequent steps. However, in this case, problems such as an increase in manufacturing cost and an increase in delivery time due to the addition of processes occur.

  Under such circumstances, methods for dealing with high-strength steel sheets exceeding existing equipment specifications with existing temper rolling mills have been studied. For example, in Patent Documents 1 and 2, as a measure of countermeasures, by increasing the work roll surface roughness, a predetermined elongation rate can be obtained without taking measures out of the range of the conventional temper rolling mill. It is disclosed that it can be granted.

  In temper rolling with a rolling reduction of about 1% or less, when rolling is performed using a roll having a large surface roughness, the portion excluded by transferring the unevenness of the roll to the surface of the steel strip appears as elongation. The phenomenon (elongation effect) becomes remarkable, and the rolling load can be reduced. According to the inventions described in Patent Documents 1 and 2, since the rolling load can be reduced by increasing the surface roughness of the work roll, a rolling load comparable to that of a soft material can be achieved without requiring large-scale equipment. A predetermined elongation can be imparted to the steel strip.

JP 2008-173684 A JP 2008-302393 A

  In the temper rolling method in the range disclosed in Patent Documents 1 and 2, it is possible to correct the shape of the high-tensile steel plate of 980 MPa class or higher while ensuring the elongation. However, there has been a problem that it is not possible to cope with high-strength steel sheets having a class of 1470 MPa or more, which have been gradually manufactured in recent years. That is, even if rolling was performed using a roll having a large surface roughness, the wave height after shape correction could not be suppressed to an allowable value or less.

  The present invention has been made in order to solve the above-mentioned problems. For high-tensile steel having a tensile strength of 1470 MPa or more, a predetermined elongation rate is ensured within the range of rolling load that can be handled by a conventional rolling mill. It aims at providing the temper rolling method of the steel strip which enables shape correction.

  In order to solve the above problems, the present inventor has intensively studied. Conventionally, when the diameter of the work roll is reduced, it has been considered that the bending of the work roll greatly affects the shape of the steel strip and it becomes difficult to correct the shape. For this reason, the diameter of the work roll has conventionally been set to 500 mm or more. However, in order to obtain the elongation required for correction even for steel plates with higher tension, the contact state between the work roll and the surface of the steel strip must be entered and exited from a completely fixed state that does not cause macro slip in the rolling direction. As a result of various investigations, it is important to shorten the contact length between the roll and the steel strip surface. I thought it was effective to make it smaller.

  That is, one aspect of the present invention is a steel characterized in that a steel strip is temper-rolled using a work roll having a diameter of 300 mm or less and a surface average roughness Ra of 3.0 to 10.0 μm. This is a temper rolling method for strips.

  Another aspect of the present invention is a work roll having a diameter of 300 mm or less and a surface average roughness Ra of 3.0 to 10.0 μm, temper-rolling a steel strip, and the work roll and the steel strip. A steel strip temper rolling facility comprising a lubricating oil supply means for supplying lubricating oil therebetween.

  When the diameter of the work roll is set to 300 mm or less, it becomes possible to correct the shape of the steel strip without requiring extensive facilities or complicated management even with high-tensile steel having a tensile strength of 1470 MPa or more.

  Further, when the surface roughness of the work roll is increased as in the present invention, the convex portion of the roll surface profile bites into the steel plate surface to serve as a wedge, and the change in the contact state is moderated. For this reason, the rolling instability phenomenon called “jumping phenomenon” can be prevented and stable rolling can be performed.

It is the figure which showed the example of a structure of the temper rolling mill suitable for implementing this invention. It is a figure which shows the result of having temper-rolled ultra-high-strength steel plate by a prior art. It is the figure which showed the result of the temper rolling of the super high strength steel plate when lubrication was added to the prior art. It is the figure which showed the result of the temper rolling of the ultra high strength steel plate when the roll diameter and the lubrication conditions were changed. It is a figure explaining the influence of the jumping phenomenon. It is the figure which demonstrated typically the influence of the roll surface average roughness Ra which exerts on a rolling load. It is the figure which showed the structural example of the conventional 4-stage temper rolling mill.

  Hereinafter, a method for temper rolling a steel strip according to an embodiment of the present invention will be described with reference to the accompanying drawings. In the temper rolling method of the steel strip of this embodiment, the elongation is 0.1% or more by using a work roll having a diameter of 300 mm or less and a surface average roughness Ra of 3.0 to 10.0 μm. The steel strip is temper rolled. The yield strength of the steel strip as the material to be rolled is 1180 MPa or more, and the tensile strength is 1470 MPa or more.

  First, the significance of using a work roll having a diameter of 300 mm or less and a surface average roughness Ra of 3.0 to 10.0 μm will be described. As shown in FIG. 7, the inventor uses a four-stage temper rolling mill described as an example in the above-mentioned Patent Documents 1 and 2 as the conventional knowledge, and has a plate thickness of 1.5 mm and a tensile strength of 1470 MPa. Attempts were made to correct the shape of the ultra-high strength steel that had been subjected to water quenching and tempering by temper rolling. Here, as with conventional knowledge, the goal was to give an elongation of 0.1%. It is because sufficient shape correction ability can be obtained if the elongation of 0.1% or more is secured. At this time, the work roll diameter was 500 mm and the unit width load was set to 10.0 kN / mm at the maximum. The surface of the work roll was processed into a surface average roughness Ra = 4.0 μm and 10.0 μm by a discharge dull processing method, and then hard chrome plating was applied.

  The result is shown in FIG. For comparison, the results of a tensile strength of 1300 MPa (Patent Document 1, Example 3) are shown in the figure. As shown in the figure, in a steel strip having a tensile strength of 1300 MPa, the temper rolling load is set to 6 kN / mm or more (when the surface average roughness Ra = 4.0 μm) or the temper rolling load is set to 4 kN / mm (surface By setting the average roughness Ra = 10.0 μm or more, the wave height after the shape correction could be reduced to 10 mm or less.

  However, in ultra-high-tensile steel with a tensile strength of 1470 MPa that exceeds the range of conventional knowledge, it is difficult to correct the shape with the conventional technology, and even if the temper rolling load is increased, the wave height after shape correction is set to 10 mm or less. It became clear that it was not possible. If the wave height after shape correction exceeds the required range of 10 mm, it will not be possible to pass the plate in the next process or later, and it will not be possible to set it at the predetermined position of the mold when press-molding the steel sheet cut out from the steel strip. May occur. The wave height was measured by placing a steel plate on a horizontal surface plate and scanning the needle of a stylus wave height meter in the width direction.

The inventor considered that this method could be straightened by further lubrication. Therefore, under the above conditions, a general tempered rolling fluid (kinematic viscosity 0.69 mm ² / s) and a cold rolling oil for emulsion (kinematic viscosity 50 mm ² / s) are each directed to the roll bite with the raw solution. The same rolling was performed while spraying from the spray nozzle. However, as a result, as shown in FIG. 3, it was found that the influence of lubrication was hardly observed. Similarly, temper rolling was performed with a cylindrical polishing roll (Rait roll in FIG. 3) of Ra = 0.2 μm, but in this case, no effect was obtained even if cold rolling oil was supplied.

  According to the inventor's study, when temper rolling is performed on ultra-high strength steel as described above, the elongation is as small as about 0.1%, so the surface between the work roll and the surface of the material to be rolled is a roll bite. It becomes a so-called fixing area on the entire inner surface and does not cause macro relative slip in the rolling direction. Therefore, the effect of lubrication cannot be obtained. This indicates that, as shown in the prior art, the method of increasing the correction force by imparting a large roughness to the surface of the work roll cannot cope with an ultra-high-tensile steel plate such as 1470 MPa class. .

  According to the inventor's study on temper rolling, the contact state between the surface of the work roll and the material to be rolled is obtained as described above in order to obtain the elongation necessary for correction even for the steel sheet having a higher tension. It was found that it was necessary to change from a completely fixed state to a state in which a relative slip zone exists near the entrance / exit side. This can be realized by shortening the contact length, and it has been found effective to reduce the work roll diameter.

  Then, the inventor performed the same rolling with the various rolling mills from which a work roll diameter differs, and compared the correction force. As a result, as shown in FIG. 4, it has been clarified that the shape can be corrected even with ultra high strength steel having a tensile strength of 1470 MPa when the roll diameter is 300 mm or less. The rolling load at this time is 10 kN / mm. In particular, when cold rolling oil is supplied, the shape becomes better. This is because slip was introduced between the surface of the work roll and the material to be rolled by the supply of the lubricating oil. Conventionally, it is known that rolling instability phenomenon called “jumping phenomenon” occurs when lubrication conditions are improved (Imai et al., Study on jumping phenomenon in wet temper rolling of annealed tin-plates with light reduction, Proc. Int. Nat. Conf. On Steel Rolling, Tokyo (1980), 1203-1214.).

  Even in the ultra-high-strength steel sheet that is the object of the present invention, as shown in FIG. 5, when the work roll surface roughness is small and lubrication is good, a rapid change in elongation occurs, and stable operation is difficult. However, in the roll roughness range defined in the present invention, even if lubricating oil is supplied, the elongation rate stably increases according to the load, so that the operation tends to be stable. According to the inventor's study, the jumping phenomenon occurs because the contact state between the roll and the material changes suddenly in the roll bite, i.e., the entire surface is fixed when the load is small and the elongation is small. The rate of elongation increases due to the rapid expansion of the sliding area. However, when the roll roughness is increased as in the present invention, the convex portion of the roll surface profile bites into the steel sheet surface to act as a wedge, and the change of the contact state is moderated, so that the jumping phenomenon is prevented. Stable rolling can be performed.

  FIG. 1 is a schematic view showing an example of a temper rolling machine to which the present invention is applied. For example, a coiler that pays out and winds a coiled steel sheet, and a temper rolling mill that is installed on the exit side of a continuous annealing facility that involves quenching and tempering processes. Ancillary equipment such as equipment for performing these heat treatments is omitted. The temper rolling method of the steel strip 4 according to the present invention provided the work roll 1 having a surface roughness Ra of 3.0 μm to 10.0 μm and a diameter of 300 mm or less as the temper rolling mill of FIG. A temper rolling facility consisting of one or more rolling stands is used. Moreover, the installation side 5 which supplies rolling oil toward a roll bite as needed is comprised at the rolling mill entrance side. This method is most effective for ultra-high-tensile steel sheets of 1470 MPa class or higher, but it is of course possible to process high-tensile steel sheets of 1300 MPa class or lower, which can be corrected by the prior art.

  In the example of FIG. 1, an example of a six-stage rolling mill is shown. The work roll 1 is elastically deformed by a rolling load. Elastic deformation of the work roll 1 is suppressed by the intermediate roll 2 and the backup roll 3. In the present invention, since the diameter of the work roll is φ300 mm or less, when a 6-stage rolling mill is used, stable rolling is easy in terms of shape and the like. However, the operation is the same for the 4-stage, 12-stage, and 20-stage types.

  The reason why the surface average roughness Ra of the work roll 1 is set to 3.0 to 10.0 μm is as follows. FIG. 6 shows the relationship between the average roughness Ra of the work roll surface and the rolling load when rolling is performed at the same rolling reduction. As shown by the dotted line in FIG. 6, for example, in normal rolling with a rolling reduction of about 5 to 50%, the rolling load with respect to the same rolling reduction increases as the average roughness of the work roll surface increases. This is because as the average roughness of the work roll surface is higher, the sliding between the steel strip 4 and the work roll 1 is suppressed, the friction coefficient is increased, the deformation of the steel strip 4 during rolling is suppressed, and the load is increased. It is. Therefore, it was common knowledge of those skilled in the art to use a bright roll having a low average roughness in order to keep the rolling load low.

  However, as a result of intensive studies by the present inventors, in temper rolling with a rolling reduction of about 1% or less, rolling is performed using a work roll 1 having a high average roughness as shown by the solid line in FIG. And newly found that the load decreases on the contrary. This is presumably because a phenomenon (hereinafter referred to as “elongation effect”) in which the portion removed by transferring the unevenness of the work roll 1 onto the surface of the steel strip appears as elongation becomes significant.

  As a result of further studies, when the average roughness Ra of the surface is about 2 μm, the unevenness of the work roll 1 pierces the steel strip 4 and the adjacent unevenness interferes with it, so that a sufficient elongation effect is obtained. It was found that could not be obtained. Therefore, it was found that the average roughness Ra of the work roll surface needs to be 3.0 μm or more in order to exert the elongation effect. In a temper rolling condition that gives a low elongation of about 0.2%, by setting the work roll surface average roughness Ra to more than 4.0 μm, the interval between adjacent convex portions becomes sufficiently large. Almost no interference of plastic deformation. Therefore, in order to effectively exert the elongation effect and reduce the load, it is desirable that the average roughness Ra of the work roll surface is more than 4.0 μm. However, it is very difficult industrially to stably perform processing with a high average roughness on the work roll 1, and it is not desirable from the viewpoint of roll life. Therefore, the average roughness Ra of the work roll surface should be 10.0 μm or less.

  Further, the steel strip 4 temper-rolled by the work roll 1 having a high surface average roughness as described above has an upper and lower surface due to the pumping effect, that is, the material movement around the indentation portion caused by local plastic deformation. Similarly, the surface shape is greatly improved by the phenomenon that the flatness is restored by shifting to a plastically stable new stress balance state. Furthermore, it has been found that the effect of shape correction becomes more prominent as the difference in average roughness of the steel strip surface before and after temper rolling, that is, the amount of increase in average roughness increases.

  The surface roughness of the work roll can be imparted by performing dull processing on the work roll surface. Here, as the method of the dull processing, a shot blast processing method, a discharge dull processing method, a laser dull processing method, an electron beam dull processing method, or the like can be used. Further, as a countermeasure against wear, a chrome plating may be applied to the roll after the dull processing.

  Here, the average roughness Ra is based on “JIS B 0601”, and only the reference length is extracted from the surface roughness curve in the direction of the average line, and the x-axis is extracted in the direction of the average line of the extracted portion. When the y-axis is taken in the direction of the vertical magnification and the roughness curve is expressed by y = f (x), the value obtained by the following equation (1) is expressed in micrometers.

  In addition, as the value of the surface average roughness Ra of the work roll 1 in the present invention, the value of Ra obtained by the above formula (1) at the representative position on the work roll surface may be used, or at a plurality of positions on the work roll surface. It is good also as a value which averaged the value of measured Ra. When using an average value of a plurality of positions, for example, at least a portion of the work roll 1 in contact with the steel strip 4 has four points at 90 ° intervals in the circumferential direction and three points at the center and both ends in the width direction. You may make it use the average value of. In the present invention, since a large roughness with an average roughness Ra of 3.0 μm or more is handled, a stable measurement value is obtained when the reference length (cut-off) is 2.5 mm and the measurement length is 12.5 mm from the JIS standard. I can do it.

  Further, in the present invention, correction is possible even in dry (processed state without supplying lubricating oil) rolling, but it is more preferable to supply lubricating oil such as tempered rolling oil or cold rolling oil. By increasing the roll roughness, stable correction is possible without causing a jumping phenomenon.

  A high strength steel plate having a thickness of 1.6 mm and a width of 1200 mm, which was quenched and tempered and had a tensile strength determined by a tensile test of 1520 MPa (yield strength 1320 MPa), was corrected by temper rolling. The steel plate after the heat treatment had a wave-like shape defect running in the rolling direction, and the maximum value of the wave height measured in the plate width direction was 25 mm.

  In the prior art, the roll surface roughness was adjusted to 9.8 μmRa by electric discharge dull processing using a four-stage temper rolling mill with a roll diameter of φ500 mm shown in FIG. Lubricating oil is not supplied.

In the example of the present invention, the roll surface roughness was adjusted to 3.2 μmRa by discharge dull processing using a 6-stage temper rolling mill having a roll diameter of 200 mm shown in FIG. Further, cold rolling oil for synthetic ester emulsion (kinematic viscosity at 40 ° C .: 50 mm ² / s) was diluted to 2%, the emulsion temperature was adjusted to 50-60 ° C., and supplied from the rolling mill inlet side. The elongation percentage was 1.2%.

  In the prior art, the load was applied to the rolling mill load limit of 16 kN / mm, but the wave height was corrected only to 15 mm and did not reach the target of 10 mm or less.

  On the other hand, in the example of the present invention, the wave height was reduced to 6 mm and the target shape was achieved with a load of 6.5 kN / mm.

  Furthermore, a high-tensile steel plate having a plate thickness of 2.0 mm and a plate width of 1500 mm, which was quenched and tempered with a tensile strength obtained by a tensile test of 1150 MPa, was corrected by temper rolling under the same conditions as described above. The steel sheet before straightening had a wavy shape defect running in the rolling direction, and the maximum value of the wave height measured in the sheet width direction was 20 mm. With the method of the present invention, the wave height was reduced to 2 mm to achieve the target shape.

DESCRIPTION OF SYMBOLS 1 ... Work roll 2 ... Intermediate roll 3 ... Backup roll 4 ... Rolled material (steel strip)
5. Lubricating oil spray nozzle

Claims (7)

  A method for temper rolling a steel strip, comprising temper rolling a steel strip using a work roll having a diameter of 300 mm or less and a surface average roughness Ra of 3.0 to 10.0 μm.   The method of temper rolling a steel strip according to claim 1, wherein the elongation percentage of the steel strip during temper rolling is 0.1% or more.   The steel strip temper rolling method according to claim 1 or 2, wherein the yield strength of the steel strip is 1180 MPa or more.   The temper rolling method for steel strip according to any one of claims 1 to 3, wherein the temper rolling is performed while supplying lubricating oil between the work roll and the steel strip.   A method for producing a steel strip, comprising temper-rolling by the method for temper rolling a steel strip according to any one of claims 1 to 4.   A steel strip having a wave height of 10 mm or less, which is manufactured by the method for manufacturing a steel strip according to claim 5. A work roll having a diameter of 300 mm or less and a surface average roughness Ra of 3.0 to 10.0 μm, and temper-rolling the steel strip;
A steel strip temper rolling facility comprising: a lubricant supply means for supplying a lubricant between the work roll and the steel strip.