JPH0347916A - Production of thick steel plate - Google Patents

Abstract

PURPOSE:To stably produce the good-quality thick steel plate which is free from internal porous shrinkage cavity by rolling the thick steel plate over two steps at different rolling shape ratios in an austenite unrecrystallization temp. at the time of producing the thick steel plate by rolling from a steel ingot. CONSTITUTION:A stock billet 2 is rolled by using rolling rolls 1 of a radius Rmm in the austenite unrecrystallization temp. region at the time of producing the thick steel plate of >=100mm thickness by hot rolling of the steel ingot or cast billet. The billet is rolled in one to several passes at <=0.4 rolling shape ratio (ld/hm) as the ratio between the contact projection length of the rolls ld and the mean plate thickness hm and is then subjected to at least 1 pass of the rolling at >0.7 rolling shape ratio at the time of rolling the billet 2 from the thickness h1 of the billet at the inlet side of the rolling rolls 1 to the thick steel plate of the thickness h2 at the outlet side of the rolls, by which the porous shrinkage cavity by the segregation in the central part are press- welded and the thick steel plate having the good quality is produced at a good yield.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industry> 2 Field of Application The present invention relates to a method for manufacturing a steel sheet, and particularly to a manufacturing method suitable for improving internal void defects in a steel sheet.

<Prior art> First of all, a steel ingot or slab has a 41° central segregation area with a high impurity flow rate in its center, and this central segregation area has a high concentration!
It is known that microscopic voids (shrinkage pores formed when molten steel solidifies) called 1 defects occur. This tendency is particularly noticeable in large steel ingots or castings with large thicknesses.

When manufacturing thick steel plates from steel ingots or slabs that have developed/jet defects, if it is possible to select a sufficient rolling reduction ratio, it is necessary to There is no problem.

<Problem to be solved by the invention> However, when manufacturing a thick steel plate with a thickness of 100 mm or more, for example, the τj method for steel ingots and 517 pieces is determined, so a sufficient rolling ratio cannot be obtained and the plate F7.11 In some cases, a pervasive defect remains in the core as an endogenous defect. Therefore, in order to obtain a sufficient rolling reduction ratio, it is necessary to increase the size of the steel ingot and increase the wall thickness of the slab, which is a problem that cannot be solved with existing equipment.

As a means to solve such problems, for example,
As disclosed in No. 7-149,122 and No. 60-56017, the difference in deformation resistance caused by forced cooling of the surface of the rolled material to create a temperature difference between the surface layer and the center of the plate thickness is A method has been proposed in which the roughness defects in the center are compressed by rolling using the same method.

However, with such means, cooling during rolling is essential, and not only does heat loss due to cooling occur,
There are problems such as rolling cracks occurring in the surface layer due to low-temperature rolling, toughness and workability being impaired due to the formation of a low-temperature transformed structure, and surface defects occurring.

Furthermore, JP-A No. 60-124401 discloses a method of reducing the rolling speed and applying strong rolling to compress the roughness defects. Since it requires a very large torque, it is undeniable that it is disadvantageous in terms of equipment, and it also has the disadvantage that rolling efficiency is hindered.

The present invention has been made to solve the problems mentioned in -1-, and is suitable for crimping internal defects such as roughness. An object of the present invention is to provide a method for manufacturing an ijm board.

<Means for Solving the Problems> The present invention provides J'/, when rolling a steel plate, in the austenite non-recrystallization temperature range, the roll contact projected length! 6 and the average plate Wh, the rolling shape ratio C1,a/
hm) is 0.4 or less, perform one pass to several passes of rolling,
Then, the rolled shape ratio is set to 0.7 or more and is at least 1.
This is a method for producing a thick steel plate characterized by rolling more than one pass.

According to the present invention, as a method of imparting a deformation resistance difference between the surface layer and the center of the rolled material, the unprocessed straw compressive force in the austenite non-recrystallized region during rolling is By utilizing the fact that strain is accumulated inside austenite grains to increase deformation resistance, the deformation resistance of only the surface layer is increased.
This was done by U.

The present inventor used a rolling row Al having a radius of R (mm) as shown in FIG. Projection length 16
The rolling shape ratio (ff
id/hm) and the compressive force at the center of the thickness of a thick steel plate that occurs when rolling with this rolling shape ratio, it was found that depending on the rolling shape ratio, the rolling edge during rolling !7 It was discovered that there are cases in which compressive force is generated at the center and cases in which it is not.

However, the reason why this rolling shape ratio (1, a/h-) is 0.4 or less in the austenite non-recrystallization temperature range for one pass or several passes is that when the rolling shape ratio is 0.4 or less, Since compressive force is generated at the center of the thickness of a thick steel plate, and only the surface layer is compressed, processing distortion in the surface layer is accelerated, and the deformation resistance only in the surface layer becomes large, and the center of the rolled material is compressed. This increases the difference in deformation resistance between the outer layer and the surface layer.

Furthermore, the reason for performing one or more passes of rolling at a rolling shape ratio of 0.7 or more is that when the rolling shape ratio is 0.7 or more -12, the rolling defects are crimped at the center of the plate thickness. The compressive force is generated by -U, and the rolling in the pre-process rolling 4; tl, 1
1 Increase the compressive force to compress the roughness defects that occur in the center of the rolled material due to the difference in deformation resistance between the center and surface layers.
This is because it promotes void defect closure.

The degree of increase in deformation resistance due to this strain accumulation is
According to the inventor's experimental results, as shown in FIG. 2, it was found that the number of rolling passes increases by a factor of 1.20 to 1.25.

<Example> Examples of the present invention will be described below.

As a result of performing ultrasonic flaw detection on a slab with a thickness of 165 II 1 m x width: 4000 mm x length: 6060 mm, made from the same steel ingot, it was confirmed that there was a roughness defect in the center of plate 17.

Five of these slabs were prepared using the method of the present invention and the conventional method.
Rolling was performed under the rolling conditions shown in the table to obtain a thickness of 100
+nm x width: 4000mm x length; 10000
mm steel plate was manufactured.

The entire surface of the thick steel plate thus obtained was inspected by ultrasonic flaw detection, and the results are shown in Table 2.

In this case, the defect occurrence rate was 0%, and it can be seen that there is no occurrence of rough defects at the center of the thickness compared to the conventional method.

<Effects of the Invention> According to the present invention, it is possible to produce an I steel plate with good wall quality without causing processing cracks or deterioration of toughness in the surface layer of the rolled material.

[Brief explanation of drawings]

Fig. 1 is a conceptual diagram of rolling to explain the average plate 17 and the projected contact length, and Fig. 2 shows the number of rolling passes and increase in deformation resistance.
It is a characteristic diagram showing the relationship with the degree of JII. 1... Rolling l]-ru, 2... Rolling cumulative month. Patent use j￥Q Hitozaki Steel Co., Ltd.

Claims (1)

[Claims] When rolling a thick steel plate, in the austenite non-recrystallization temperature range, the roll contact projected length l_d and the average plate thickness h_m
The rolling shape ratio (l_d/h_m) expressed as the ratio of
．． 4. A method for manufacturing a thick steel plate, characterized in that rolling is performed for one pass to several passes at a rolling shape ratio of 0.4 or less, and then rolling is performed for at least one pass or more at a rolling shape ratio of 0.7 or more.