JP2001334306A - Method of manufacturing hot-rolled steel strip - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a hot-rolled steel strip for suppressing the generation of surface flaws and securing the lower limit of finishing temperature which is necessary to obtaing mechanical properties of not lower that the SPHD class. SOLUTION: The whole width direction of a material to be rolled is heated before finish rolling and, when performing finish rolling, the scale on the material to be rolled by jetting high-pressure water in at least one interstand where the passing time of the material to be rolled between adjacent stands is not less than 1 s. Furthermore, it is preferable to cool the surfaces of the material to be rolled with water in all interstands where the passing time is not less than 2 s.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for producing a hot-rolled steel strip. More specifically, the present invention relates to a method for producing a hot-rolled steel strip having excellent surface properties and excellent mechanical properties.

[0002]

2. Description of the Related Art Generally, a hot-rolled steel strip is roughly rolled from a slab heated to a predetermined temperature by a rough rolling mill to form a material to be rolled, and scale is removed by spraying high-pressure water onto the surface of the material to be rolled. Then, finish rolling is performed by a finish rolling mill having a plurality of stands to produce the product.

[0003] The temperature of a hot-rolled steel strip on the exit side of a finishing mill (hereinafter, also referred to as a finishing temperature) greatly affects the mechanical properties of the hot-rolled steel strip. Therefore, a lower limit of the finishing temperature (hereinafter, also referred to as a finishing lower limit temperature) is determined in order to secure desired mechanical properties.

By the way, in the production of the hot-rolled steel strip,
Since the material to be rolled is transported and rolled in a high temperature state, an oxide scale is generated on the surface of the material to be rolled. This oxide scale is mainly composed of iron oxide, but may contain metal oxide.

[0005] When hot rolling is performed while such an oxide scale (hereinafter, also simply referred to as a scale) is present on the surface of a material to be rolled, the scale is pushed into the surface during rolling and surface flaws are generated. In particular, in recent years, with the improvement of the mechanical properties of the hot-rolled steel strip, the range of its use has been expanded, and the need for high surface quality has increased, and prevention of surface quality deterioration due to scale has become important. For example, as a cold-rolled alternative hot-rolled steel sheet, a hot-rolled steel strip having a sheet thickness of 2 mm or less, having mechanical properties of SPHD class or higher, and having good surface properties is required.

As measures for preventing surface quality deterioration due to scale, JP-A-6-269840 and JP-A-10-12
No. 8423 discloses a method in which after the surface of a steel sheet is heated on the entry side of a finish rolling mill, descaling is performed with high-pressure water and finish rolling is immediately performed.

[0007]

However, even in the method disclosed in the above-mentioned publication, particularly when a hot-rolled steel strip having a small thickness is rolled, surface flaws are generated due to the remaining fine secondary scale. And cannot meet the needs of high surface quality.

On the other hand, as a measure against the secondary scale, a method of performing descaling with high-pressure water between the stands of the upstream stands of the finishing mill having a long passage time between stands and a large rolling load can be considered. However, when descaling is performed by injecting high-pressure water between stands, particularly when rolling a hot-rolled steel strip having a small thickness, the temperature of the material to be rolled is greatly reduced, and it is difficult to secure a minimum finishing temperature. Mechanical properties may not be obtained.

Therefore, it is difficult to stably roll a hot-rolled steel strip having a finish plate thickness of 2 mm or less, a mechanical property of SPHD class or more, and excellent surface properties by the conventional technology. there were.

An object of the present invention is to suppress the occurrence of surface flaws,
It is another object of the present invention to provide a method for manufacturing a hot-rolled steel strip capable of securing a minimum finishing temperature necessary for obtaining mechanical properties of SPHD class or higher.

[0011]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have conducted various studies and obtained the following findings. (A) By performing a heat treatment for heating the entire width of the material to be rolled before the finish rolling, it is possible to secure a finish lower limit temperature. However, due to a rise in the temperature of the material to be rolled, the generation of scale between the stands of the finishing mill increases, and in particular, surface flaws occur during rolling on the upstream stand where the passage time of the material to be rolled between adjacent stands is long. Likely to happen.

(B) By removing the scale by spraying high-pressure water onto the surface of the material to be rolled between stands where the passage time is 1 second or longer, the occurrence of the surface flaw is suppressed. (C) Therefore, a heat treatment for heating the entire width of the material to be rolled before the finish rolling, and a material to be rolled between the stands of the finishing mill in which the passage time of the material to be rolled between adjacent stands is 1 second or more. By removing the scale by injecting high-pressure water onto the surface of the surface, it is possible to secure the minimum finishing temperature and suppress the occurrence of surface flaws. Steel strip can be manufactured.

(D) When the passing time is 2 seconds or more,
Due to the increase in the amount of scale generated between stands, high Si
In hot-rolled steel strips such as steel and high Mn steel, scale removal between stands may be insufficient.

(E) By spraying water on the surface of the material to be rolled at the outlet of the upstream stand between the stands to cool the surface of the material to be rolled, the amount of scale generated between the stands can be reduced. .

(F) Therefore, scale removal between the stands is performed, and further, low pressure water is applied to the material to be rolled at the outlet of the upstream stand between all the stands in the finishing mill where the passage time between the stands is 2 seconds or more. And cooling the surface of the material to be rolled, the generation of surface flaws can be suppressed even when rolling a hot-rolled steel strip such as a high Si steel or a high Mn steel.

The present invention has been completed based on the above findings, and the gist thereof is as follows. (1) From upstream to downstream, a rough rolling mill, a heating device,
A descaling device and a finish rolling machine are sequentially provided, and the slab is subjected to rough rolling by the rough rolling machine to obtain a material to be rolled, and then the entire width direction of the material to be rolled is heated by the heating device. Removing high-pressure water by a device to remove the scale of the material to be rolled, and then performing finish rolling with the finish rolling mill having a plurality of stands to produce a hot-rolled steel strip, wherein the finish rolling is performed. Manufacturing a hot-rolled steel strip, wherein high-pressure water is injected between at least one stand having a passage time of a material to be rolled between adjacent stands of the mill of 1 second or more to remove scale of the material to be rolled. Method.

(2) An inter-stand descaling device is provided between a plurality of stands where the passing time is 1 second or more, and according to the target value of the finished plate thickness of the material to be rolled and the target value of the lower limit temperature of the finish. The method for manufacturing a hot-rolled steel strip according to the above (1), wherein the number of bases used in the inter-stand descaling device is adjusted.

(3) A cooling device is provided at the outlet of the upstream stand between all the stands where the passing time is 2 seconds or more, and the surface of the material to be rolled is cooled with the cooling device. The method for producing a hot-rolled steel strip according to the above mode (1) or (2).

The present invention is suitable for a method for producing a hot-rolled steel strip that can be used as a cold-rolled alternative steel sheet having a finished plate thickness of 2 mm or less. Further, the invention described in the above item (3) is suitable for producing a hot-rolled steel strip of a high Si-containing steel having a high Si content or a hot-rolled steel strip of a high Mn-containing steel having a high Mn content. .

[0020]

Embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a schematic diagram schematically showing a main part of an example of a hot rolling facility for carrying out the present invention.

As shown in FIG. 1, this hot rolling equipment
A finishing mill 5 comprising a heating device 1, a heating device 2 and seven stands (F1 stand to F7 stand from upstream to downstream), and a descaling device provided on the entry side of the F1 stand, that is, before finishing. A descaling device 4, a descaling device provided between stands F1 to F5, that is, inter-stand descaling devices 6a, 6b, 6c, 6d, and a cooling device provided between stands F1 to F3. 8
a, 8b. Reference numeral 3 denotes a crop shear, 7 denotes a finishing thermometer, 10 denotes a material to be rolled, and 11 denotes a hot-rolled steel strip.
Hereinafter, scale removal by the pre-finishing descaling device and the inter-stand descaling device will be referred to as pre-finishing descaling and inter-stand descaling, respectively, and cooling by the cooling device will also be referred to as inter-stand cooling.

The slab heated to a predetermined temperature is subjected to rough rolling by a rough rolling mill comprising one or more stands to become a material to be rolled 10 (hereinafter, also referred to as a rough bar). Then
As shown in FIG. 1, the coarse bar 10 is transported on a roller table and heated to a predetermined temperature by a heating device 1 and a heating device 2 installed in front of an F1 stand. In accordance with the above, the leading and trailing ends of the rough bar 10 are cut, and thereafter, high-pressure water is sprayed on the surface by the pre-finishing descaling device 4 to remove scale, and the finishing mill 5
The hot rolled steel strip 11 having a predetermined thickness is rolled.

The heating device 1 is a full width heating device for heating the entire width direction of the rough bar 10, and for example, a solenoid type induction heating device is used. The heating device 2 is an edge heating device for heating the width end portion, and for example, an induction heating type heating device is used. In FIG. 1, the heating device 2 is disposed downstream of the heating device 1, but the heating device 2 may be provided upstream of the heating device 1. However, from the viewpoint of ensuring the finishing temperature, in order to efficiently raise the temperature of the width end having a large temperature drop, it is preferable that the heating device 2 be installed downstream of the heating device 1 as shown in FIG.

In the heating of the material to be rolled by the heating device 1 and the heating device 2, the finishing temperature in all the regions in the longitudinal direction and the width direction is equal to or higher than the finishing lower limit temperature determined according to the mechanical properties of the hot-rolled steel strip as the product. In consideration of the pre-finishing descaling, the stand-to-stand descaling, and the temperature drop due to the stand-to-stand cooling, the temperature of the material to be rolled is increased by combining the heating device 1 and the heating device 2. Hereinafter, the amount of temperature rise of the material to be rolled by the heating device 1 and the heating device 2 is referred to as a temperature rising amount.

FIG. 2 is a graph showing an example of a change in the temperature of the material to be rolled and the temperature of the hot-rolled steel strip on the exit side of the F7 stand, ie, the finishing temperature, in the longitudinal direction. FIG. 2B shows the change in the amount, and the change in the finishing temperature is shown by converting the position in the longitudinal direction on the horizontal axis into the value of the coarse bar length.

As shown in FIGS. 2A and 2B, the finishing temperature can be controlled substantially uniformly in the longitudinal direction by adjusting the amount of temperature rise of the material to be rolled in the longitudinal direction. In addition,
Hereinafter, the maximum value of the heating amount in the longitudinal direction is referred to as the maximum heating amount.

The amount of temperature rise of the material to be rolled is larger at the width direction ends than at the width center portions, and the temperature distribution in the width direction at the finishing mill entry side is higher at the width direction ends than at the width center portions. It is desirable to heat to. This is because the temperature drop of the material to be rolled due to the injection of high-pressure water increases at the width end.

As the pre-finishing descaling device, a conventional high-pressure water injection type descaling device may be used.
The apparatus has a discharge pressure of about 5 to 20 MPa. In order to reduce the temperature drop of the material to be rolled during high-pressure water injection, it is preferable to increase the pressure and reduce the amount of water to be injected.

The coarse bar from which the scale has been removed by the pre-finishing descaling device is subjected to finish rolling sequentially from the F1 stand. At this time, in order to remove the scale formed on the surface of the rough bar, the inter-stand descaling device is used. And inject high pressure water. In the embodiment of FIG. 1, the case where the inter-stand descaling devices 6 a to 6 d are provided between the stands of the F1 stand to the F5 stand is shown. The scale may be removed by injecting high-pressure water between at least one stand for more than one second.

The scale formed on the surface of the material to be rolled varies depending on the finished plate thickness and the finished temperature. That is, as the thickness of the finished plate becomes smaller and the finishing temperature becomes higher,
Scale is easy to generate. Therefore, it is desirable to provide a stand-to-stand descaling device between a plurality of stands and adjust the number of bases of the stand-to-stand descaling device to be used according to the finishing plate thickness and the finishing temperature.

For example, in accordance with the finishing plate thickness and the finishing temperature, the number of bases to be used by the inter-stand descaling device and the stand between which the scale is to be removed are determined in advance for each steel type, and this is tabulated, and based on this table, What is necessary is just to perform descaling between stands.

When the passage time between the adjacent stands increases, the amount of scale generated between the stands increases, so that the scale removal between the stands by the injection of high-pressure water becomes insufficient. Hot-rolled steel sheets such as high-Si-rich steel and high-Mn-rich steel with a high Mn content tend to have surface flaws.

Therefore, a cooling device is provided at the outlet of the upstream stand between all the stands where the passing time between the adjacent stands is 2 seconds or more, and the cooling device injects cooling water to the material to be rolled, and It is desirable to cool the surface.
In the embodiment of FIG. 1, the cooling device 8a is provided between the F1 stand to the F3 stand where the passage time is 2 seconds or longer.
8b. Note that the discharge pressure of the cooling water may be about 2 MPa or less.

By cooling the surface of the material to be rolled between stands where the passage time is 2 seconds or more, the amount of scale generated between stands can be suppressed, and the pressure between the stands caused by injection of high-pressure water can be reduced. The effect of scale removal is improved. Therefore, by cooling the surface of the material to be rolled between the stands, it becomes possible to produce a hot-rolled steel sheet of high Si steel or high Mn steel having good surface properties.

In the description of the embodiment, a hot rolling facility provided with a heating device 1 as a full width heating device and a heating device 2 as an edge heating device is used as a heating device for a material to be rolled. However, the present invention is not limited to this mode, and is similarly applied to a hot rolling facility having only a full-width heating device as a heating device.

[0036]

(Example 1) A rough bar having a thickness of 30 mm and a width of 1250 mm was finish-rolled by a hot rolling facility having the basic structure shown in FIG. Steel strip. The steel type is low carbon steel. The minimum finishing temperature is SPHD
The temperature was set to either 820 ° C. required to obtain a hot-rolled steel strip having mechanical properties of a class or 840 ° C. required to obtain a hot-rolled steel strip having mechanical properties of a SPHE class.

Using the induction heating type heating devices 1 and 2, the width of the coarse bar is controlled while controlling the amount of temperature rise of the coarse bar in the longitudinal direction so that the finishing temperature is in the range of the lower limit of finishing temperature to the lower limit of finishing temperature + 20 ° C. The entire direction was heated.

High-pressure water was sprayed on the surface of the material to be rolled using at least one of the stand-to-stand descaling devices 6a to 6d of high pressure water injection type provided between the stands F1 to F5. That is, based on the passage time of the material to be rolled between the adjacent stands shown in Table 1, high-pressure water was injected between at least one stand having a passage time of 1 second or more to remove scale.
The discharge pressure of the high-pressure water was 14 MPa.

[0039]

[Table 1] As a comparative example, the condition without heating of the rough bar and no descaling between stands (Comparative Examples 1 and 4) and the condition with heating of the rough bar and no descaling between stands (Comparative Examples 2, 3 and 4) 5) was also carried out.

The finishing temperature was measured over the entire length of the hot-rolled steel strip by the finishing thermometer 7 provided on the exit side of the F7 stand to determine the minimum value of the finishing temperature, and the surface properties of the hot-rolled steel strip were examined. Table 2 shows the minimum value of the finishing temperature and the surface properties of the hot-rolled steel strip together with comparative examples. In addition, in the same table, the maximum heating amount indicates the maximum value of the heating amount of the coarse bar in the longitudinal direction at the time of heating the rough bar by the heating devices 1 and 2.

[0041]

[Table 2] As shown in Table 2, in all of the examples of the present invention, the scale flaw occurrence rate is less than 5%, and the minimum finishing temperature 82 over the entire length.
It was possible to secure 0 ° C. Therefore, it was found that the method of the present invention enables the production of a hot-rolled steel strip having good surface properties and mechanical properties of SPHD class or higher.

In the comparative example, the scale defect generation rate was 5% or more and the surface properties were poor. Further, in Comparative Example 1, there is a part where the finishing temperature is lower than 820 ° C.
It was found that a hot rolled steel strip having mechanical properties higher than HD class could not be obtained. Example 2 A hot-rolled steel strip having a thickness of 30 mm and a width of 1250 mm was formed into a 1.4 mm-thick steel strip by the hot rolling equipment having the basic configuration shown in FIG. Steel type is high Si content steel (S
i: 0.3% by mass). The finishing lower limit temperature was set to 830 ° C. which is necessary for obtaining a hot-rolled steel strip having mechanical properties of SPHD class or higher.

In the same manner as in the first embodiment, the finishing temperature is set at the finishing temperature to the finishing lower temperature +20 in the heating devices 1 and 2 of the induction heating system.
The entire width direction of the coarse bar was heated while controlling the temperature rise amount of the coarse bar in the longitudinal direction so as to be in the range of ° C. Next, high-pressure water was injected by a pre-finishing descaling device to remove scale, and then finish rolling was performed.

In the finish rolling, the F1 stand to F4
Using at least one of the inter-stand descaling devices 6a to 6c provided between the stands of the stands, F1 in which the passing time shown in Table 1 is 1 second or more
At least one between each stand from stand to F4 stand
High-pressure water was injected between the two stands to remove the scale. The discharge pressure of the high-pressure water was 14 MPa.

Further, in Example 2 of the present invention, the above-mentioned scale removal is performed, and between the stands where the passage time is 2 seconds or more, that is, between the stands of the F1 stand and the F2 stand and between the stands of the F2 stand and the F3 stand. Cooling water was injected by cooling devices 8a and 8b provided at the outlet of the upstream stand to cool the surface of the material to be rolled. Note that the discharge pressure of the cooling water was 2 MPa.

As a comparative example, the test was performed under the conditions of no descaling between stands and no cooling between stands. In the same manner as in Example 1, the finishing temperature was measured over the entire length of the hot-rolled steel strip by the finishing thermometer 7 provided on the exit side of the F7 stand to determine the minimum value of the finishing temperature, and the surface properties of the hot-rolled steel strip were measured. investigated.

Table 3 shows the minimum finishing temperature and the surface properties of the hot-rolled steel strip together with comparative examples.

[0048]

[Table 3] As shown in Table 3, it was found that the examples of the present invention had good surface properties and could secure a finishing temperature of 830 ° C. or higher.
In particular, in Example 2 of the present invention in which inter-stand cooling was performed, no scale flaw was generated and the surface properties were extremely good. On the other hand, the comparative example was poor in surface properties.

[0049]

According to the present invention, particularly when rolling a thin hot-rolled steel sheet having a thickness of 2 mm or less, scale flaws generated on the surface of the hot-rolled steel strip can be suppressed, and a high finishing temperature can be secured. Therefore, it becomes possible to produce a hot-rolled steel strip having excellent surface properties and mechanical properties equal to or higher than the SPHD class. Further, according to the present invention, by cooling the material to be rolled between the stands, the amount of scale generated between the stands is suppressed, and a hot rolled steel strip of high Si steel or high Mn steel having excellent surface properties can be obtained. Manufacturing becomes possible.

[Brief description of the drawings]

FIG. 1 is a schematic diagram schematically showing a main part of an example of a hot rolling facility for carrying out the present invention.

FIG. 2 is a graph showing an example of a change in a heating amount of a material to be rolled and a temperature of a hot-rolled steel strip on an exit side of an F7 stand, that is, a change in a finishing temperature in a longitudinal direction. (B) is a change in the finishing temperature.

[Explanation of symbols]

 1, 2: heating device, 3: crop shear, 4: descaling device before finishing, 5: finishing rolling mill, 6a to 6d: descaling device between stands, 7: finishing thermometer, 8a, 8b: cooling device, 10 : Rolled material (rough bar), 11: Hot rolled steel strip.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) B21B 45/02 320 B21B 37/00 132B

Claims (3)

[Claims] 1. A rough rolling mill, a heating device, a descaling device, and a finish rolling mill are sequentially provided from upstream to downstream, and the slab is subjected to rough rolling by the rough rolling mill to obtain a material to be rolled, After the entire width direction of the material to be rolled is heated by a heating device, high-pressure water is injected by the descaling device to remove the scale of the material to be rolled, and thereafter, finishing is performed by the finishing mill having a plurality of stands. A method for producing a hot-rolled steel strip by performing rolling, wherein high-pressure water is injected between at least one stand where the passage time of a material to be rolled between adjacent stands of the finishing mill is 1 second or more. A method for producing a hot-rolled steel strip, comprising removing scale from a material to be rolled. 2. A stand-to-stand descaling device is provided between a plurality of stands having a passage time of 1 second or more, and the target value of the finished plate thickness of the material to be rolled and the target value of the minimum finish temperature are set according to the target. The method for producing a hot-rolled steel strip according to claim 1, wherein the number of bases used in the inter-stand descaling device is adjusted. 3. A cooling device is provided at an outlet of an upstream stand between all the stands where the passage time is 2 seconds or more, and the surface of the material to be rolled is water-cooled by the cooling device. Or the method for producing a hot-rolled steel strip according to 2.