JPS59215208A - Method for preventing scale pattern of thick steel plate - Google Patents

Abstract

PURPOSE:To reduce the build-up of scale pattern by controlling the temperature of a sheet bar to a specific value and descaling the bar in the vicinity of a final rolling pass under the conditions given by specifying the relations between the impact pressure of water jet and the time of descaling. CONSTITUTION:In the vicinity of the final pass of rolling process of a thick steel plate; a descaling by water jet is applied to a sheet bar controlled to a 850-1,100 deg.C temperature range under the conditions given by an equation III(contents of S in steel >=0.010wt%) and an equation IV (contents of S<= 0.010wt%), which specify the relations between a water-jet impact pressure Pa expressed by an equation I [P: descaling pressure (kg/cm<2>), alpha: nozzle discharging angle (deg.), f: projecting length (cm) at the colliding surface of water jet, H: height of nozzle (cm)] and a descaling time (t) [N: the number of nozzles, V: transferring speed (cm/S) of plate].

Description

[Detailed Description of the Invention] Technical Field Regarding the suitability of descaling for methods for preventing scale patterns on thick steel plates The following information is related to descaling control in thick plate rolling aimed at avoiding scale patterns that occur in thick steel plates. Therefore, it belongs to the technical field related to plate rolling.

Problem: When manufacturing thick steel plates in a point plate mill, there are
An uneven scale defect with a depth of approximately 0.01 to 0.0511 may occur.

This is called a scale pattern, and the cause is that the secondary scale layer generated during plate rolling was not completely removed and remained despite the high-pressure water injection from the mill's descaling nozzle. This is because a partial secondary scale layer bites into the entire surface layer of the steel substrate during rolling, and then falls off during polishing such as shot blasting, resulting in depressions.

In order to prevent this scale pattern, it is necessary to almost completely remove the secondary scale layer generated during rolling during descaling before the final rolling pass.

Conventionally, the following points have been considered as a method for removing this secondary scale.

■ Increase the descaling pressure.

■ Increase the number of rows of descaling equipment.

■ To prevent nozzle clogging of descaling equipment ■
In the method of increasing the descaling pressure, the amount of electricity increases exponentially as the pressure increases, leading to a significant increase in cost. The method (2) of increasing the number of rows of deske-1 nongs is impractical in the case of thick plate mills because there is no place to install them. ■
The method for preventing nozzle clogging is an indirect means to compensate for the effects of (1) and (3) above, and this method alone cannot provide the essential solution fcv to the above-mentioned problem 15.On the other hand, recently due to demands from customers, There are many cases where high-injection steel is required, and accordingly the S content -1)E is reduced, but at this time there is a strong tendency for scale patterns to occur frequently. However, the chemical components in steel, especially the S content and the peeling properties of secondary scales during thick plate rolling, have not been fully elucidated until now. The following experiment was conducted to understand the effect on scaling performance (one unit).

Experiment 1 Using 41k41/m1'' class shipbuilding material as the test material, heating conditions and rolling conditions were kept constant, and chemical components, especially S.
The effects of the content and the threading speed during descaling on the incidence of scale patterns were investigated.

Although there are a wide variety of factors that affect the descaling property, the experiment was conducted by adjusting the threading speed of thick steel plates, which is easy to control, and fixing the other factors.

It goes without saying that the lower limit of the plate passing speed is limited by the rolling speed of the plate rolling mill. Now, water gene as a factor affecting descaling property) I
The fli thrust pressure pa is expressed by the following equation (1).

Pa=/(p, α, b, H) −...= <1
In formula 3, p: descaling pressure C#/CIrL") α: nozzle discharge angle C66g) b: projected length of water jet on the impact surface cIJ'L) H: nozzle height ((1 m)

t=/(N, V)...In formula (2), N = number of nozzles V: Transport speed of thick steel plate (Crn/s), especially f... We found that scale patterns can be effectively avoided under conditions that follow the formula.

J pa x t ≧15
M view % J] i × t≧20 X 10-8 (I
cgy2-8/cm) =-(,3゛°,°steel medium S
According to the content (0,010% by weight, ie (3) above), (according to the three formulas), the scale pattern occurrence rate can be suppressed to about 8% to a little over 3%.

Experiment 2 Next, we investigated the effect of descaling temperature on scale exfoliation rate and obtained the results shown in Figure 2.

Here, the sample material is shown as an example in which s: o, oos% is reduced, but generally it is 850°C or higher, at most 1100°C.
A sufficiently high peeling rate can be achieved within the range of .

Purpose of the Invention According to the results of the above experiments, it is an object of the present invention to provide appropriate descaling conditions depending on the S content, thereby reducing the scale pattern occurrence rate.

Constitution of the Invention It can be said that it is clear from what has already been said that the above object can be effectively achieved by satisfying the following matters.

Near the final pass of the rolling process of thick steel plate, the above formula (31
, C8') is applied to a sheet bar whose temperature is controlled in the range 1... of 850° to 1100°C, to lead to a reduction in the scale pattern occurrence rate. Method for preventing scale patterns on thick steel plates.

The scale pattern prevention behavior with the above configuration is as described in Examples.

Example 1 41 to 4 containing 5% by weight of SO in the component composition
1/, □ 1000 pieces of 2nd class shipbuilding material (thickness 25mm x width 2000mm x length 7000mm) were rolled at a hot finishing temperature of 950°0 according to the usual method, and the descaling conditions in the final pass of the rolling process were was as follows.

P = 150 kfl/am”, α = 15°, b =
8α, I (=40 crrL, therefore pa is 1.1
19 kg/crrL20 Also, N=tko, V=ax4m/s and 6D, therefore t
is 0. θ was 255s.

Thus, the descaling factor in this invention is Ji
x t = 27 x 100-8tc%''/cIn.

At this time, out of 1,000 thick steel plates tested, the number of scale pattern occurrences was 80, so the scale pattern occurrence rate was suppressed to 8%.

Comparative Example 1 The same number of test materials as in Example 1 were descaled in the same manner, except that the descaling conditions were changed to v-near 54 crIL, /H. At this time, the descaling factor rX t according to the present invention is 11.2 X 1O-8 (k41
%-s,,Q), the number of scale pattern occurrences was 98 out of 1000 sheets, and therefore the scale pattern occurrence rate was as high as 9.8%.

Example 2 4. S content in the component composition was 0.016%. x/cg/a-
Class shipbuilding materials (thickness 25 mm x width 2000 mm x length 7 mm)
m), 1000 sheets were hot rolled according to a conventional method, the hot finishing temperature was 950''C, and the descaling conditions for the final pass of the rolling process were as follows.

P = 1.50 #I/, p, α=15°, b=8
I, H=40cIn, so pa is 1, 119
1cg/cIIL2 0 Also, N = 1 piece, v; 58oa/8, so t is 0
．． U15 Eir:, 9.

Thus, the descaling factor in this invention is J″
'\Lxt=16xto-8tcg3Aψs4.

At this time, out of 1000 thick steel plates tested, 28 plates had scale patterns, so the scale pattern occurrence rate was 2.8%.
was suppressed.

Comparative Example 2 The same test material as in Example 2 was descaled in the same manner as in Example 2, except that the descaling conditions were changed to V=754 Cm/b. At this time, Tesuku according to this invention.

-Ring factor νξix t t4.11.2 X 10
-' tcg8-8/Cm, test thick steel plate 10
Out of 00 sheets, 50 sheets had scale patterns, so the scale pattern occurrence rate was 5.0%.

Comparative Example 3 As a conventional method, 41 kg/, with an S component composition of 0.018%,
, Z class shipbuilding materials (thickness 20m 7RX width 2000mm x length 7
000) iooo sheets were rolled according to a conventional method at a hot finishing temperature of 950°C, and in the final pass of the rolling process, P = 150#I/crn21 α = 15°, b
=Bcm, H=4041. Therefore Pa = 1.11
It was 9 klj/,,2.

Also, N = 1 piece * V = 754 cm/'6 r.
(Descaling was performed under the condition of t=0.0118.

Applying this conventional method to the descaling factor related to this invention, 5 x t = 11 x 10-8 kg
%°S/, L, and the number of scale pattern occurrences is 55 out of 100 test materials, so the scale pattern occurrence rate is 5.5
%Met.

Table 1 also shows a comparison of the number of scale pattern occurrences and the scale pattern occurrence rate for the above-mentioned Examples and Comparative Examples.

Table 1 Comparison of scale pattern occurrence rate In the above comparative example, especially when the S content of the thick steel plate is low, the descaling condition is 4x t.

However, in Example 1.2 according to the present invention, not only when the S content was 0.016%, but also when the S content was 0.016%. In particular, in the case of 0.005%, the problem of staying in the United States could be resolved.

In the case of the embodiment of the present invention, the scale pattern occurrence rate is reduced by at least approximately half compared to the case of the comparative conventional method, and the effect of preventing scale patterns is large.

Effects of the Invention As described above, according to the present invention, by optimizing the descaling conditions, the secondary scale layer generated on the front and back surfaces of thick steel plates with a particularly low S content can be advantageously removed, resulting in fewer scale patterns. Thick steel plates with surface texture can be manufactured.

[Brief explanation of drawings]

Figure 1 is a graph showing the influence of descaling pressure or descaling factors on the scale pattern occurrence rate of a thick steel plate, depending on the amount of S content in the steel plate. 1 is a graph showing the effect of descaling temperature on peeling rate. Patent applicant: Kawasaki Steel Corporation

Claims (1)

[Claims] When near the final pass of the rolling process of L thick steel plate, the following formula (1
), the water jet impingement pressure pa q is expressed as below (
2) The following (
8) (Descaling by water jet under the conditions given by the 8 formulas was applied to a sheet bar whose temperature was controlled in the range of 850° to 1100°C, leading to a reduction of the scale pattern occurrence rate by 1. A method for preventing scale patterns on thick steel plates. Pa= f (P, a, b, H) - (1
) where P: nib scaling (kg/, i) α: nozzle discharge angle (dc4) b Projected length of industrial water jet on impact surface CIW) H: nozzle height crrL) t=/(N, V) Curve・(2) In the formula, N = number of nozzles, ■ = transfer speed of thick steel plate (c1rL/s) J Pa
×1. ≧15
pa×t, ≧20×1010-8('-s/cIn
)--(8'f°, ° S content in steel (0,010% by weight