JP2004001019A - Method for preventing hardness variation of hot-rolled coil - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method that prevents periodic hardness variation generated at the end of a hot-rolled coil and that reduces thickness deviation caused in a cold-rolled steel plate. <P>SOLUTION: A coil car 2 that transports a hot-rolled coil 1 wound at 400-700°C to a placing table is heated at 80°C or above in the contact part 4 with the hot-rolled coil 1, thereby suppressing martensitic transformation caused by locally cooled outer circumferential part of the hot-rolled coil. In addition, it is even more desirable that the coil placing table is also heated or insulated in the contact part with the hot-rolled coil or that the hot-rolled coil placed on the coil placing table is covered with a heat-retaining cover. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for preventing fluctuation in hardness of a hot-rolled coil wound at 400 ° C. or higher.
[0002]
[Prior art]
The hot-rolled steel sheet is wound around a mandrel in a temperature range of 400 ° C. to 700 ° C. to become a hot-rolled coil. The hot-rolled coil is pulled out of the mandrel by a coil car, transferred to a coil holder at a coil storage place, and naturally cooled. . This hot rolled coil has a length of, for example, 600 m.
[0003]
However, at the end of the steel sheet on which the hot-rolled coil is cold-rolled, a phenomenon that the thickness deviation at the tail part becomes large as shown in FIG. 7 may occur. This phenomenon is particularly remarkable for high strength steel. Analysis of the state of occurrence indicates that periodic hardness fluctuations occur at the portion corresponding to the end of the hot-rolled coil, and this hardness fluctuation increases the thickness deviation of the cold-rolled steel sheet. Was confirmed.
[0004]
[Problems to be solved by the invention]
The present invention has been made to solve the conventional problems described above and to provide a method for preventing a hardness change of a hot-rolled coil that can prevent a periodic hardness change occurring at an end of the hot-rolled coil. is there.
[0005]
[Means for Solving the Problems]
In order to solve the above problems, the present inventors further pursued the cause. As a result, the hardness variation was large at two locations on the outer peripheral side of the hot-rolled coil, and particularly in high-tensile steel, martensitic transformation occurred in these portions. The hardness was found to be higher. Moreover, it turned out that those positions correspond with the contact part with the hot rolled coil of a coil car.
[0006]
The present invention has been made based on the above findings, and heats a contact portion with a hot rolled coil of a coil car for transferring a hot rolled coil wound at 400 ° C. or more to a coil holder, thereby forming a hot rolled coil. It is characterized in that local martensitic transformation at the outer periphery is suppressed. In addition, it is preferable to heat the contact portion of the coil car with the hot rolled coil to 80 ° C. or higher. Further, the effect of the present invention can be further enhanced by heating or insulating the contact portion of the coil holder with the hot rolled coil, or by covering the hot rolled coil placed on the coil holder with a heat insulating cover.
[0007]
ADVANTAGE OF THE INVENTION According to this invention, it suppresses that the outer peripheral part of a hot-rolled coil of high temperature is locally cooled by contact with a coil car, and suppresses the martensitic transformation which tends to generate | occur | produce especially in a high strength steel, and prevents hardness fluctuation. it can. Since the hot-rolled coil may be locally cooled even after being transferred to the coil holder, the contact portion of the coil holder with the hot-rolled coil is heated or insulated, or placed on the coil holder. More favorable results can be obtained by applying a heat insulating cover to the placed hot rolled coil.
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described.
The hot-rolled coil of this embodiment contains C: 0.05 to 0.2% (mass%, the same applies hereinafter), Si: 0.4 to 2%, and Mn: 0.5 to 4%. The coil is made of a steel containing at least one of Mo, Ti, Nb, Ca, and Cu, and has a tensile strength of 800 MPa or more. This steel sheet is wound around a mandrel 2 as shown in FIG. The winding temperature range is 400 ° C. to 700 ° C. as in the conventional case.
[0009]
Reference numeral 3 denotes a coil car for extracting the hot-rolled coil 1 from the mandrel 2 and transferring it to a coil holder at a coil storage. The coil car 3 has V-shaped receiving bases on both sides of the upper surface. The coil car 3 enters from the standby position to immediately below the hot-rolled coil 1, ascends, and is removed from the mandrel 2 as shown in FIG. 1. As shown in FIG. 2, at this time, the V-shaped pedestal becomes the contact portion 4 with the hot-rolled coil 1.
[0010]
In the present invention, by heating the contact portion 4 with the hot-rolled coil 1, the outer peripheral portion of the hot-rolled coil 1 is prevented from being locally cooled, and martensitic transformation due to cooling is suppressed. As a heating method, an arbitrary method such as a method in which a heater is inserted into the pedestal and direct heating is performed, and a method in which heating is performed indirectly by a radiant heating unit at a standby position indicated by an imaginary line in FIG. 1 can be adopted.
[0011]
In order to suppress the martensitic transformation of the ultra-high strength steel, the entire hot-rolled coil 1 wound at 400 to 700 ° C is cooled at a rate of 100 ° C / h or less in a temperature range of 500 to 200 ° C. It is necessary to heat the contact portion 4 of the coil car 3 with the hot-rolled coil 1 to 80 ° C. or more. By heating the contact portion 4 with the hot-rolled coil 1 in this manner, local cooling of the outer peripheral portion of the hot-rolled coil 1 can be prevented, and the martensitic transformation is suppressed and the hardness does not fluctuate. Can be.
[0012]
As described above, the local cooling by the contact portion 4 of the coil car 3 which is a main factor of the hardness variation can be prevented, but the hot-rolled coil 1 extracted from the mandrel 2 by the coil car 3 is placed in the coil holder as shown in FIG. Since it is placed on the platform 5, it is also cooled by the contact portion 6 of the coil platform 5 with the hot-rolled coil 1. Therefore, it is preferable to heat or insulate the contact portion 6 of the coil holder 5 with the hot-rolled coil 1.
[0013]
The heating method is the same as that of the contact portion 4 of the coil car 3 described above, and a heater may be inserted into the inside to directly heat or may be indirectly heated in advance by radiant heating means. Since the temperature of the hot-rolled coil 1 is considerably lowered when transferred to the coil holder 5, the heating temperature may be lower than that of the contact portion 4 of the coil car 3, and only the heat insulating material 7 is interposed as shown in FIG. May be. Further, as shown in FIG. 5, a heat insulating cover 8 may be placed on the hot-rolled coil 1 placed on the coil holder 5, and the whole may be insulated to reduce the cooling rate.
[0014]
FIG. 6 is a graph showing the thickness deviation of a steel sheet obtained by cold rolling a hot-rolled coil extracted from a mandrel by the coil car 3 in which the contact part 4 is heated to 100 ° C. Compared to FIG. 7 showing a conventional example using a coil car in which the contact portion is not heated, it can be seen that the thickness deviation near the end is reduced by a fraction. This material is the above-mentioned ultra-high tensile steel, and the total length of the cold-rolled steel plate is 1200 m. It is considered that if measures such as heating, heat insulation, and heat retention are also applied to the coil holder 5, the thickness deviation is further reduced.
[0015]
【The invention's effect】
As described above, according to the method for preventing a hardness change of a hot-rolled coil of the present invention, the outer peripheral portion of the hot-rolled coil is prevented from being locally cooled by contact with the coil car, and the martensitic transformation is prevented. And fluctuation in hardness can be prevented. For this reason, the thickness deviation of the cold-rolled steel sheet manufactured from the hot-rolled coil can be significantly reduced as compared with the related art. Further, if measures such as heating, heat insulation, and heat retention are taken after the hot-rolled coil is placed on the coil holder as in claims 3 and 4, the thickness deviation can be further reduced.
[Brief description of the drawings]
FIG. 1 is a perspective view showing a coil car.
FIG. 2 is a front view showing a hot-rolled coil conveyed by a coil car.
FIG. 3 is a front view showing a hot-rolled coil placed on a coil holder.
FIG. 4 is a front view showing the insulated coil holder.
FIG. 5 is a front view showing a hot-rolled coil kept warm by a heat retaining cover.
FIG. 6 is a graph of a thickness deviation of a cold-rolled steel sheet showing the effect of the present invention.
FIG. 7 is a graph of a thickness deviation of a conventional cold-rolled steel sheet.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Hot-rolled coil 2 Mandrel 3 Coil car 4 Contact part of coil car with hot-rolled coil 5 Coil holder 6 Contact part of coil holder with hot-rolled coil 1 Insulation material 8 Heat insulation cover

Claims (4)

The hot rolled coil wound at 400 ° C. or higher is heated at a contact portion with a hot rolled coil of a coil car that transfers the hot rolled coil to a coil holder, thereby suppressing local martensitic transformation of the outer peripheral portion of the hot rolled coil. To prevent fluctuation in hardness of hot rolled coil. The method according to claim 1, wherein a contact portion of the coil car with the hot-rolled coil is heated to 80 ° C. or higher. The method for preventing a hardness change of a hot-rolled coil according to claim 1, wherein a contact portion of the coil holder with the hot-rolled coil is heated or insulated. The method for preventing hardness fluctuation of a hot-rolled coil according to claim 1, wherein the heat-retaining cover is hung on the hot-rolled coil placed on the coil holder.

Images