JPH0569001A - Manufacture of extremely thick steel plate - Google Patents

Abstract

PURPOSE:To continuously manufacture a thick steel plate excellent in internal quality from a slab by completely press-welding porosities even in rolling at low rolling reduction ratio of the specified rolling reduction ratio of 1.5-2.0. CONSTITUTION:When the front and back surfaces of the slab is water cooled during rolling after uniformly hating the slab 1 which is manufactured by continuous casting method at a temp. of >=1250 deg.C and the slab is rolled by making a temp. difference between those front and back surfaces and the center part of thickness, cooling is controlled so that the front and back surfaces in the range of the middle part 3/4 to the entire width of the slab have a temp. difference of >=200 deg.C between the center part in the thickness direction. After that, by executing rolling of >=2 passes that rolling shape ratio of >=0.40/pass is ensured on the basis of the max. rolling reduction that is bearable to the capacity of the rolling mill, a extremely thick steel plate with press-welded porosities that are generated in the vicinity of the center of the thickness of slab is manufactured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to an extremely thick steel plate which is obtained by subjecting an extremely thick slab immediately after continuous casting to water cooling on the front and back surfaces, and performing strong reduction rolling while making a temperature difference between the front and back surfaces and the center in the thickness direction. Manufacturing method.

[0002]

2. Description of the Related Art Porosity, which is called a Zaku flaw, which occurs at the end of solidification exists near the center of the thickness direction of a slab manufactured by continuous casting. Usually, this porosity is pressure-bonded during the plate rolling process. In an extremely thick steel plate with a small reduction ratio from the slab thickness to the product plate thickness, this porosity remains and becomes a steel plate with poor internal quality that is detected by inspections such as ultrasonic flaw detection tests of the product.

As a method for improving this, a strong reduction rolling method of rolling with a large reduction amount per pass is adopted. Between the slab surface and the center portion in the thickness direction in order to carry out reduction to the slab center portion more effectively. It has been proposed to apply a temperature difference rolling method in which a temperature difference is applied to the sheet, and a method using this combination. For example, by applying a temperature difference of 400 ° C. or more and securing a rolling shape ratio of 0.5 or more, the center porosity of the continuous casting slab is pressure-bonded (see Japanese Patent Publication No. 61-238404), and the temperature difference during extraction is predicted and extracted. Controlling air-cooling and water-cooling conditions from rolling to rolling, and improving the end surface shape and internal quality by rolling with a temperature difference (JP-B-5)
There is a prior art such as Japanese Patent Laid-Open No. 4-68760).

[0004]

By the way, in order to make maximum use of the milling capacity, only the cooling range in the width direction, that is, the range in which the center porosity occurs is sufficient, and this control is effective. There is no known technique in this regard, and it is known that the improvement effect cannot be expected so much even with the above-described prior art, especially when rolling at a low pressure ratio.

An object of the present invention is to reduce the continuous casting slab to a reduction ratio.
An object of the present invention is to provide a method for manufacturing an extra-thick steel sheet capable of press-bonding porosity even at a low pressure ratio of 1.5 to 2.0 and sufficiently improving the internal quality.

[0006]

The present invention, in order to achieve the above object, is characterized by having the following constitution. That is, the present invention, the slab manufactured by the continuous casting method, during the rolling after uniformly heating at a temperature of 1250 ℃ or more in a continuous heating furnace or batch type heating furnace, water cooling the front and back of the slab, When rolling with a temperature difference from the center in the thickness direction, center 3/4 of the full width of the slab
Cooling control is applied to the front and back surfaces in the range of above so that there is a temperature difference of 200 ° C or more from the central portion in the thickness direction, and then a rolling shape ratio of 0.40 / pass or more based on the maximum reduction amount that can withstand the rolling functional force. By performing rolling with more than two passes, the extra-thickness steel plate is produced in which the porosity generated near the center of the slab thickness is pressure-bonded even in the rolling range where the reduction ratio from the slab is 1.5 to 2.0. It is a method for manufacturing an extremely thick steel plate.

[0007]

The porosity at the center of the slab does not occur in the entire widthwise direction of the slab, but normally occurs in the center 3/4 of the entire width. On the other hand, the amount that can be rolled down in one pass due to the capacity of the rolling mill is determined by the temperature of the slab and the width at the time of rolling, so it is better to keep the temperature of the slab as high as possible. However, cooling more than necessary is effective. Will be damaged.

In the present invention, the width to be cooled is confirmed in advance, and the temperature difference between the slab surface and the central portion in the thickness direction is 20.
Cooling to 0 ° C. or higher, and then, as rolling immediately after water cooling, maximizing the milling capacity and rolling at least two passes at a rolling reduction ratio of 0.4 or more. As a result, crushing of porosity in the central portion of the rolled thick steel sheet can be reliably performed, and a thick steel sheet with excellent internal quality can be manufactured. Further, it is possible to manufacture a thick steel plate having excellent internal quality even in a low pressure reduction rolling with a reduction ratio of 1.5 to 2.0 from a slab.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a continuously cast slab 1 according to the present invention.
FIG. In this slab 1, a portion shown as 3/4 W, that is, a portion corresponding to the central portion 3/4 width with respect to the entire width is the water cooling range, and porosity is generated in the center portion in the thickness direction of this range. FIG. 2 is a manufacturing flow of the method of the present invention. In the first step A1, the slab 1 is heated in a continuous heating furnace or a batch heating furnace at a temperature of 1250 ° C. or higher for 6 hours or more, and is heated uniformly to the center of the slab 1 in the plate thickness direction. After that, the process proceeds to step A2, and after extraction from the furnace, the scales on the front and back surfaces generated in the furnace are completely removed by a scale breaker.

Then, the process proceeds to step A3, and the rolling mill is rolled for one pass under a light pressure such that the unevenness of the surface of the slab 1 is eliminated, for example, under a pressure of 10 mm to smooth the surface shape and prevent uneven cooling due to water cooling. .. Continue to step A
4, the surface of the smoothed material is uniformly water-cooled using a water cooling device such as a shower, and a temperature of 200 ° C. or more is applied to the surface and the back surface of the slab 1 as the material and the center in the plate thickness direction. Cooling is performed for about 1 minute until a difference is given. The cooling range of this water cooling in the width direction of the slab 1 is about 3/4 of the width center portion where the center porosity is generated.
Control the cooling range.

After cooling with water, the process proceeds to Steps A5 and A6, and strong reduction rolling with a rolling shape ratio of 0.40 / pass or more, for example, 40 mm reduction rolling is continuously performed for at least two passes or more. Then, the process proceeds to step A7, and the product is rolled to a predetermined product thickness.

A product of 180 mm thickness according to the present invention, which was cooled and rolled based on the above manufacturing flow, and a product of the same shape manufactured by the conventional method were compared with each other by an ultrasonic flaw detection test according to JIS standard. Inventive products are acceptable products with no defects due to porosity, whereas comparative products are J products.
According to ISG0801, a defect of (F / B)> 50% occurred and it was unacceptable.

[0013]

As is apparent from the above description, the predetermined rolling is performed such that a temperature difference of 200 ° C. or more is produced between the front and back surfaces of the slab and the central portion in the thickness direction at the place where porosity occurs. Even in the low-pressure reduction rolling with a reduction ratio of 1.5 to 2.0 from the slab, the porosity is completely pressure-bonded, and it is possible to continuously manufacture thick steel plates with excellent internal quality.

[Brief description of drawings]

FIG. 1 is a perspective view of a continuous casting slab 1 according to an embodiment of the present invention.

FIG. 2 is a manufacturing flow of the method of the present invention.

[Explanation of symbols]

 1: Slab.

Claims (1)

[Claims] 1. A slab manufactured by a continuous casting method is uniformly heated in a continuous heating furnace or a batch heating furnace at a temperature of 1250 ° C. or higher, and then, during rolling, the front and back surfaces of the slab are water-cooled to obtain the thickness of the front and back surfaces. When rolling with a temperature difference from the center of the slab, the front and back surfaces in the center 3/4 of the entire width of the slab are controlled to have a temperature difference of 200 ° C. or more from the center of the thickness direction, After that, rolling is performed in two or more passes with a rolling shape ratio of 0.40 / pass or more based on the maximum amount of reduction that can withstand the rolling functional force, so that the rolling ratio from the slab is 1.5 to 2.0. A method for producing an extra-thick steel sheet, which comprises producing an extra-thick steel sheet in which porosity generated near the center of a slab thickness is pressure bonded.