JPH04238620A - Method for descaling in hot rolling - Google Patents

Abstract

PURPOSE:To easily and surely remove the scale of hard-to-peel property from a hot rolling steel plate which can not be peeled off conventionally and to prevent the generation of the scale flaw on this hot rolling steel plate. CONSTITUTION:In manufacturing the hot rolled steel plate by hot rolling, the high pressurized water is sprayed on the surface of the steel plate before finishing rolling, whose high pressurized water spray is, at a colliding pressure per unit spraying area >=20(g/mm<2>), <=40(g/mm<2>), and flow rate >=0.1{l/(min.mm<2>)}, <=0.2{l/(min.mm<2>)}.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] In order to produce hot-rolled steel sheets with good surface properties, the present invention is designed to remove difficult-to-remove scales such as red scales and embedded scales that cause scale defects before finish rolling. The present invention relates to a descaling method during hot rolling in which water jets are used to remove descaling.

0002

[Prior Art] In the production of hot-rolled steel sheets by hot rolling, a raw material slab is charged into a heating furnace with an oxidizing atmosphere, heated at 1100 to 1450°C for several hours, and then passed through a rough rolling mill and a finishing rolling mill. However, if the primary scale generated on the slab during heating or the secondary scale generated after primary scale removal is not sufficiently peeled off during rolling, the scale will dig into the surface of the product and cause scale. It remains as a flaw. If such scale flaws occur, the surface quality will be significantly impaired, and they will become a starting point for cracks during bending, etc., resulting in serious harm to the quality of the product.

[0003] Usually, in order to prevent the occurrence of such scale flaws, a rolling line with a pressure of about 100 to 150 [kg/c] is applied.
water jet descaling device (
A descaler (hereinafter referred to as a descaler) is installed, and this is used to peel off and remove scale on the surface of the steel sheet while rolling. However, the reality is that the quality of scale removability is strongly influenced by the operating conditions of the descaler as well as the properties of the scale, i.e. its composition and structure, and in particular the primary scale of steel with a high Si (silicon) content. It is known that it becomes extremely difficult to peel off in some cases. The reason for this is that Si contained in the steel undergoes selective oxidation during high-temperature oxidation during heating, and 2FeO, SiO, which has high thermoplasticity,
2 (fairlite), and a subscale layer with a unique structure with a complicated interface with the metal is formed. For example, when steel containing 0.1% or more of Si is heat treated, the formation of the subscale develops significantly, resulting in countless scale defects remaining on the surface of the product after rolling, which significantly reduces the commercial value of the product. It may reduce the

[0004] Regarding secondary scale that occurs after primary scale removal, if extremely thick scale is unevenly formed due to scratches on the steel plate, the scale cannot be sufficiently removed under normal descaling conditions. As a result, scale defects often occur.

[0005] Techniques for avoiding such problems include:
In Japanese Patent Publication No. 1085/1985, “Si 0.10 to 4
．． When manufacturing a hot rolled steel plate by hot rolling a slab made of steel containing 0.00%, the cumulative reduction rate calculated from the start of rolling is 65% and the billet temperature is 1000°C or higher within the rolling period. , 80-250 [kg/cm2
] is applied for a cumulative time of 0.04 seconds or more, using a high-pressure water jet.
Descaling method during hot rolling of steel. ' has been proposed. The key points are the following two points.

(1) The descaling properties of the eutectic of fairlite and wustite have a remarkable temperature dependence, and there are temperature ranges where descaling is easy and temperature ranges where it is not. That is, in "Tetsu to Hagane" lecture summary collection 1982 S439, FET (Finisher Entry Temper
ature (temperature immediately before finish rolling) is approximately 930°C,
It is described that it has the worst scale removability. Therefore, in order to avoid this region with poor peelability, the FET was
00℃ or higher.

(2) Fairlite generated during the heating of the slab invades the base steel in a eroded manner and has a complex interfacial structure. It is impossible to peel and remove this by descaling with high pressure water. However, as the rolling reduction progresses, the erosion zone becomes raised on the surface of the steel plate, so that the descaling property improves at a cumulative rolling reduction rate of 65% or more. That is, it is most effective to descale with high pressure water immediately before finish rolling.

[0008]

[Problems to be Solved by the Invention] However, regarding the method for preventing scale flaws in Si-containing steel disclosed in the above-mentioned Japanese Patent Publication No. 1085/1985, in order to perform rolling under operating conditions that satisfy the above-mentioned points, the following steps must be taken. There are problems like this.

(1) Since it is necessary to secure a high temperature FET, the actual FET must be set at a much higher temperature than the lower limit FET allowed by the material, and for this reason, high temperature heating is required when heating the slab. As a result, the unit heat consumption increases and the production yield decreases due to an increase in scale loss.

(2) Since various constraints are added to the rolling reduction rate and descaling time, the rolling operation becomes complicated.

(3) In the case of thin slab casting and direct rolling, since there is no rolling before finishing rolling, no improvement effect on scale properties due to rolling can be expected.

The present invention was made in order to solve the above-mentioned conventional problems, and it is possible to easily and reliably remove difficult-to-remove scale without imposing various restrictions on heating temperature, rolling reduction rate, descaling time, etc. - The purpose is to provide a descaling method during hot rolling that can remove

[0013]

[Means for Solving the Problems] The present invention provides that when producing hot rolled steel sheets by hot rolling steel materials, the collision pressure per unit spread area is 20 [g/mm2] or more and 40 [g/mm2] or more before finishing rolling. g/mm2 ] or less, and the flow rate is 0.1 [
l / (min ・mm2)] or more 0.2 [l / (
The above objective was achieved by injecting a high-pressure water spray of the following amount (min·mm2) onto the surface of the steel plate.

[0014]

[Function] The present inventors conducted detailed research experiments regarding the descaling properties of primary scales generated during hot rolling of Si-containing steel, which is difficult to peel off scales, and obtained the following knowledge.

[0015] The experiment was carried out using the F of an actual hot strip mill.
SB (Finisher Scale Breaker)
: In the high-pressure water descaling device immediately before finish rolling), the amount of high-pressure water spray per unit area can be reduced by varying the supplied high-pressure water pressure, the spread angle and thickness of the water spray, and the distance between the nozzle and the steel plate surface. The collision force Pt and the flow rate V were varied to examine various degrees of occurrence of red scale. As a result, it is possible to simply spray high-pressure water without being subject to various constraints such as the high-pressure water supply pressure, descaling time, steel plate temperature during descaling, and cumulative reduction rate of the steel plate until descaling as in the conventional method. It has been found that the removability of red scale (primary scale) can be improved by appropriately selecting the collision pressure Pt per unit spread area and its flow rate V at the time of collision with a steel plate.

The present invention was created based on the above knowledge, and the reasons for limiting the constituent elements (reasons for restricting conditions) in the present invention will be explained in detail below.

In the present invention, the impact pressure Pt of high-pressure water spray
from 20 [g/mm2] to 40 [g/mm2]
] or less, and the flow rate V is 0.1 [l / (min ・m
m2)] or more than 0.2 [l/(min・mm2)
] It is important to regulate the following.

FIG. 1 is a graph for explaining the reason for the above regulation, in which the vertical axis is the spray impact pressure Pt [g/m
m2], and the horizontal axis is the spray water flow rate V [l/(min
・mm2)]. Also, in the diagram, ◎ and ○
The mark indicates a good product, and the mark △ and × mark indicate a defective product because there is a problem with the product.

The above-mentioned FIG. 1 shows that a slab made of steel A having the composition shown in Table 1 is used as the steel material, and a steel plate formed through a predetermined rough rolling process is rolled into, for example, F1 to F7 as shown in FIG.
When finishing rolling with a finishing rolling mill, descaling is performed by changing the impact pressure Pt on the steel plate S of the spray sprayed from the descaling header of the FSB installed immediately before the finishing rolling mill and its flow rate V, and then This figure shows the results of investigating the extent of scale flaws in a hot-rolled product (steel plate S) when finish rolling is performed. Steel having the composition shown in Table 1 has a property that the primary scale is particularly difficult to peel off.

[0020]

[Table 1]

As is clear from FIG. 1, when the collision pressure Pt is 20 [g/mm2] or more and the flow rate V is 0.1 [g/mm2] or more,
l/(min ・mm2)] or more,
There are almost no scale flaws caused by red scale (primary scale), which has extremely poor removability, and exhibits excellent descaling properties.

[0022] Then, the collision pressure Pt is 20 [g/mm
2 ], the flow rate V is 0.2 [l/(min ・
When the flow rate V is 0.1 [l/(min ・mm2)], there are no scale defects when the collision pressure Pt is 40 [g/mm2]. .

Therefore, the spray impact pressure Pt is set to 40 [
g/mm2] or the spray flow rate V is greater than 0.2 [l/(min ・mm2)],
The effect of preventing scale defects caused by red scale does not improve any further, and on the contrary, the descaling equipment and its ancillary equipment become more complex and larger, resulting in higher equipment and operating costs, as well as worsening maintainability. Since this occurs, the impact pressure P of the spray during descaling is
t is 20 [g/mm2] or more and 40 [g/mm2]
], the flow rate V is 0.1 [l/(min ・mm
2)] or more than 0.2[l/(min・mm2)]
It turns out that it is important to do the following:

[0024]

EXAMPLES Next, examples of the present invention will be described in detail.

Steels B to G with chemical compositions as shown in Table 2
The same procedure as in the case of steel A shown in Table 1 was carried out, except that the operating conditions were set as shown in Table 3 (No. 1 to 17) and Table 4 (No. 18 to 35) for the slab consisting of After descaling, finish rolling was performed, and the incidence of scale defects in the obtained products is also shown in Tables 3 and 4.

[0026]

[Table 2]

[0027]

[Table 3]

[0028]

[Table 4]

[0029] In Tables 3 and 4, No. 1 for steel B. 1
As can be seen from 28, if the collision pressure Pt and flow rate V are within the range of the present invention (Nos. 1 to 7), scale defects will not occur, but if they are outside the range of the present invention (Nos. 8 to 12).
) (Nos. 24 to 28) etc., scale flaws were noticeably generated.

[0030] Also, No. From 13 to 23, the collision pressure Pt
And the rationality of setting an upper limit to the flow rate V is understood.

Furthermore, the occurrence of scale flaws is caused by the impact pressure Pt
It is clear that it depends only on the conditions of Pt and flow rate V, and does not depend on other factors such as high water supply pressure, descaling temperature, descaling time, and cumulative reduction rate, and the impact pressure Pt and flow rate during spraying As long as V is within the range of the present invention, for example, No. It can also be seen that the occurrence of scale flaws can be prevented even when the rolling reduction ratio is 0 as shown in No. 6.

[0032] The above-mentioned effects are as follows. As is clear from Nos. 29 to 35, it can be seen that the same results can be obtained even when the chemical composition of the steel is different.

Although the present invention has been specifically explained above, the present invention is not limited to what has been shown in the above-mentioned specific examples, and can be modified in various ways without departing from the gist thereof.

[0034] In the above specific example, the case of steel with a composition of less than 0.1 Si was not shown, but steel with a composition of less than 0.10% Si
It is clear that in hot-rolled steel sheets with a diameter of less than 100 mm, less scale is generated during slab heating, and therefore descaling can be easily performed using the present invention.

[0035]

[Effects of the Invention] As described above, according to the present invention, when hot-rolling a hot-rolled steel plate, difficult-to-peel scale that could not be peeled off in the past can be reliably removed without being subjected to various operational constraints. As a result, it is possible to prevent scale flaws from occurring in the hot rolled steel sheet.

[Brief explanation of the drawing]

FIG. 1 is a graph for explaining the effects of the present invention.

FIG. 2 is a schematic explanatory diagram showing an example of a finishing rolling mill.

[Explanation of symbols]

F1 to F7...finishing rolling mill, FSB...high pressure water descaling device, S...hot rolled steel plate.

Claims (1)

[Claims] Claim 1: When manufacturing hot rolled steel sheets by hot rolling steel materials, before finishing rolling, the impact pressure per unit spread area is 2.
0 [g/mm2] or more and 40 [g/mm2] or less, and the flow rate is 0.1 [l/(min・mm2]
)] to 0.2 [l/(min·mm2)] High-pressure water spray is injected onto the surface of a steel sheet.