JPH0255605A - Manufacture of very thick steel plate of excellent internal quality - Google Patents

Abstract

PURPOSE:To effectively reduce UST defects by performing plural rolling passes by setting finish rolling speeds in a specific speed range and drafting a stock in the casting thickness direction by a shape ratio not less than a specific value for all the rolling passes. CONSTITUTION:A billet is subjected to width spread rolling in rough rolling stages and then is rolled to have a product thickness in finish rolling stages. In finish rolling stages, plural rolling passes are performed at a rolling speed in 200-350mm/sec and the billet is drafted in the thickness direction by a rolling shape ratio M (roll projected contact length/average sheet thickness between roll gap) of >=0.5 for all the passes. An equivalent diameter of a center porosity and a billet thickness before rolling are denoted by d0 and h0, respectively; the d0 is changed to an equivalent diameter dk after K passes and delta shows a pressure welding progressing degree by plastic deformation of the stock. Progressing degrees are proportional to drafting capability of a rolling roll and plastic deformation progresses with a contacting time of a rolled stock with a roll, so that low speed rolling is effective in this method. Hence, UST defects in the steel are sheet effectively reduced by a combination of the low speed rolling with large drafting rolling and a very thick steel plate are easily manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for rolling an extra-thick steel plate having a product thickness of 80 mm or more, in which the material to be rolled is a slab produced by a continuous casting method.

[Prior art]

Conventionally, the final solidification position of slabs obtained by continuous casting is at the center of the slab, so center segregation and center porosity, which are unique to continuous casting, are unavoidable at the center of the slab. In particular, center porosity is a minute void, and if it is not crimped during the rolling process, the product will fail the shipping inspection due to a UST defect. Current continuous casting methods allow slab thicknesses up to approximately 300 mm, but in order to completely compress the center porosity, it is necessary to apply sufficient reduction during the rolling process. A slab obtained by continuous casting with a product plate thickness of 80 mm or more cannot be sufficiently rolled down, making it impossible to manufacture a product free of UST defects. For this reason, in the conventional method of manufacturing extra-thick steel plates, as shown in Japanese Patent Application Laid-Open No. 62151201 and Japanese Patent Publication No. 62-13083,
The material to be rolled is a material rolled by the steel ingot method.

A problem with such conventional techniques is that the production cost using the steel ingot method is several thousand yen higher per ton of steel material than that using the continuous casting method, which is disadvantageous in terms of cost.

On the other hand, in the method of manufacturing extra-thick steel plates using slabs produced by the continuous casting method, as disclosed in Japanese Patent Publication No. 62-54561, ``The material to be rolled is placed on the inlet side of the mill line of a thick plate rolling mill to reduce its wall thickness. There is a "thick plate rolling equipment" which is equipped with a forging press that rolls down in the direction of U.
This suggests that it is impossible to manufacture extra-thick steel plates without using a forging press when ST defects are taken into account.

The problem with such forging press equipment is that compared to rolling equipment having rolls, it requires more expensive equipment and has higher utility costs such as electric power.

In order to solve the above-mentioned problems of the conventional technology, Japanese Patent Application Laid-Open No. 61-23840 is proposed as a method that does not require capital investment.
As shown in Publication No. 4, "Steel materials with a temperature difference of 400°C or more between the surface and the center have a shape ratio (longitudinal effective contact length (
There is a "hot working method for steel materials" which is characterized by applying a reduction in the thickness direction and/or width direction at a ratio of 0.5 to 0.5. On the one hand, it has the advantage of not requiring capital investment, but on the other hand, it is a steel material with a temperature difference of 400"C or more between the surface and the center." 40
In order to provide a temperature difference of 0° C. or more, water cooling is required on the surface of the slab, which increases the loss in unit heat consumption.

(2) In order to perform large reduction rolling with a shape ratio of 0.5 or more when the surface of the slab is 400 t: or more lower than the center, the rolling capacity of the rolling mill must be higher than that of normal rolling. It takes several times more. Therefore, a large capital investment is required to increase the capacity of the rolling mill.

There is a problem.

[Problem to be solved by the invention]

The present invention solves the above-mentioned problems of the prior art in a method of manufacturing extra-thick steel plates by continuous casting, that is, high costs and large capital investments due to the manufacturing method.

[Means to solve the problem]

The present invention advantageously solves the problems of the prior art described above and has the following features.

That is, in a method for manufacturing an extra-thick steel plate in which a continuous casting slab is tentered in a rough rolling process and further rolled to the product thickness in a finish rolling process, the rolling speed is set at a rolling speed of 200 to 350 R/min in the finish rolling process. The method is characterized in that a plurality of passes are rolled at sec, and further, in all of the above-mentioned passes, reduction is applied in the thickness direction of the slab with a shape ratio of 0.5 or more.

Hereinafter, the content of the present invention will be specifically explained.

A slab (slab thickness of about 300 mm) is produced by a continuous casting method, and then heated in a heating furnace or directly rolled in a rough rolling mill. Rolling by this rough rolling mill is mainly for tentering rolling, and multiple passes are rolled, and the thickness of the rolled material is approximately 1.5 to 2 times the product plate thickness (e.g., 100 to 150 Tsuru). , about 200 to 250 layers). after that,
The finish rolling mill performs multiple passes of rolling to reach the product thickness, and the rotational speed of the rolls is 200 to 350.
Low speed rolling of 1 m/sec is required. In order to carry out such low-speed rolling, the backup roll bearings in the finishing rolling mill must be motorized oil film bearings.
Rolling bearings with excellent load-bearing performance are most preferred. When the rotational speed of the roll is less than 200i+■/sec,
The contact time between the material to be rolled and the roll becomes long, and the roll itself tends to suffer from heat cracks and rough skin due to heat load when using a rolling roll, so the lower limit of the rotational speed of the roll is set to 200/sec. In addition, if the roll rotation speed exceeds 350 In/sec, the effect of low-speed rolling cannot be obtained, and for example, when rolling with a strength of 50 to 80 kg/+n 2 class is performed in a pressure vessel for an electronic power equipment chemical plant, finishing rolling may be difficult. The rotation speed of the roll of the machine is 350mm.
/sec, UST defects will occur in the product even if the shape ratio is increased (approximately 1 to 3). Therefore, the upper limit of the rotation speed of the rolls of the finishing rolling mill is set to 350 mm/sec.

Here, we will further explain the necessity of low speed rolling in the finish rolling mill. As multiple passes are rolled in the rolling process, it is most desirable that the center porosity gradually decreases to the point where it is crimped. However, as the center porosity gradually decreases, the load from the rolling rolls should be reduced around the center porosity. As a result, plastic deformation occurs. As this plastic deformation progresses, the center porosity is efficiently compressed.

The progress of the plastic deformation described above is proportional to the rolling capacity of the rolling rolls, or when the rolling capacity of the rolling rolls is the same, the progress of the plastic deformation increases as the contact time between the rolled material and the rolls increases. In order to efficiently reduce the center porosity while crimping the material without increasing the rolling capacity of the rolling rolls, low-speed rolling is extremely effective as a method of increasing the contact time between the material to be rolled and the rolls.

Next, large reduction rolling in a finish rolling mill, which is another feature of the present invention, will be explained. That is, in the present invention, the rolling shape ratio (M,) is limited under the conditions of low speed rolling as an index of high reduction rolling. Rolling shape ratio M for each pass is M, = 1d/hI11 Mi ; Rolling shape ratio ld ; Roll projected contact length (fin) hm ; Average sheet thickness in roll gap (crane) R: Roll diameter
(Dragon) h,: plate thickness < i = 0.1.2...) (ho: rolling oil plate thickness) (h,: plate thickness after i passes after rolling) i: number of rolling passes, If all of the rolling shape ratios M8 are less than 0.5, the center porosity decreases in proportion to the decrease in plate thickness in the finish rolling process, but crimping is not achieved even if the number of passes is increased. The reason for this is that tensile stress acts on the center of the steel material and no compressive stress acts on it, so even if the number of passes is increased, it is not enough to crimp the center porosity. Therefore, the lower limit of the rolling shape ratio M is set to 0.5. On the other hand, the upper limit of the rolling shape ratio Ml is about 3, considering that the thickness of the slab that can be manufactured by the current continuous casting method is about 300 to 350 mm, and the product thickness of extra-thick steel plate is 100 to 2001 mm.

Next, we will explain in detail the technical content of crimping the center porosity inside the slab in the continuous casting method by combining the basic techniques of low-speed rolling and high-reduction rolling, which are the characteristics of the present invention. If the equivalent diameter of the center porosity before rolling of a continuous casting slab is do, and the equivalent diameter of the center porosity after K-pass rolling is dK, then d*/do is:
f (Mi); porosity compressive stress function g (Vi)
; Porosity crimping speed influence coefficient i ; Number of rolling passes (
1,...K) ho ; Thickness before rolling (1 plate) hK i Thickness after rolling (, m). However, M=<0.5 m/sec,
When V,';=0.5m/sec, equation (2) is dx/do #hx/ha
(3) becomes. FIG. 1 is a diagram showing the center porosity shape before and after rolling. FIG. 1(a) shows the center porosity before rolling, where the equivalent diameter is dll and the slab thickness is h0. Fig. 1(b) shows the center porosity after pass rolling, and the equivalent diameter dK is δ, which is the contact length and contact time between the roll and the rolled material due to low speed rolling and large reduction rolling. This shows the degree of crimping progress due to plastic deformation of the material, which occurs due to a large value.

Normally, when the rolling speed is 0.5 m/sec or more, δ-〇, and as shown in equation (3), the equidistant diameter ratio dK/do of the center porosity before and after rolling is the thickness ratio hK of the rolled material before and after rolling. /ho, so theoretically the center porosity cannot be eliminated by compression. The content of equation (2) will be explained regarding the center porosity crimping effect due to low speed rolling and large reduction rolling, which are the characteristics of the present invention.

Figure 2 shows the rolling shape ratio M, and the porosity pressure stress number f.
(M,), both of which are expressed by the following equation (
5).

f (member t)=aM,” +bM□+c (
5) From the above formula, the porosity pressure stress function f (M,
) is an increasing function of the rolling shape ratio M, where the rolling reduction in one pass is small and in the region of M+<0.4, the porosity compression effect due to the rolling shape ratio is very small, and f(Mi)=0.

Figure 3 shows rolling speed V and porosity crimping speed influence coefficient g
(Vi), both of which are expressed by equation (6).
It is related to

Vaporosity thickness decreases with decreasing strain rate, and
Bonding of the porosity inner surface is promoted as the crimping time increases. That is, the porosity crimping speed influence coefficient g (Vi) is expressed as a decreasing function of the rolling speed V. Further, this low speed effect g(Vi) promotes porosity crimping as a synergistic effect with the large rolling effect f (ML), and the low speed effect is small in a region where the rolling shape ratio is small.

Figure 7 shows the rolled shape ratio M! In the case of 0.5 (in the case of the present invention)
The relationship between the rolling speed and the residual porosity thickness ratio dK/d0 is shown. At a rolling speed of 0.35 m/see or less, the residual porosity thickness dK becomes O, which clearly represents the synergistic effect of the large rolling effect f (Mi) and the low speed effect g (Vi).

That is, it specifically indicates the validity of equation (4).

〔Example〕

(1) Continuously cast common steel slabs having a slab thickness of 300 μl were passed through a rough rolling mill for tenter rolling, and then passed through a finishing mill for finishing rolling at a temperature of 900°C.

The rolled product size is 1200 (thickness) x 2800 (
width) (dragon). The finish rolling speed is 300 mm/sec in the method of the present invention, and 2000 mm/sec in the comparative example.
It was sec.

The results are shown in FIG. 4 in terms of the relationship between plate thickness and rolling shape ratio.

According to this, the method of the present invention was able to obtain a larger rolling shape ratio per pass than the comparative example, and in combination with the low speed rolling effect, a rolled product with excellent internal properties was obtained.

(2) Continuously cast special steel slabs with a slab thickness of 300 mm (low-temperature toughness like marine construction materials) are passed through a rough rolling mill for tentering rolling, and then passed through a finishing mill at a temperature of 750°C. Finish rolling was performed. Finished product rolling size is 100 (thickness) x 25
It was 00 (width) (crane). The finish rolling speed is 300 mm/sec in the method of the present invention, and 200 mm/sec in the comparative example.
011/sec.

The results are shown in FIG. 5 in terms of the relationship between plate thickness and rolling shape ratio.

The material of the present invention has a low rolling temperature due to the need for controlled rolling, which is disadvantageous for large reduction rolling, but Example 1
As with the general rolling, it was possible to increase the rolling shape ratio per pass compared to the comparative example, and in combination with the low speed rolling effect, a rolled product with excellent internal properties was obtained.

That is, the internal properties of this example are shown in FIG. 6 together with a comparative example. The rolling conditions for the comparative example of the present invention were as follows.

The above figure shows the effect of low speed rolling of the present invention and the synergistic effect of the combination of low speed rolling and high reduction rolling, and shows how effective the present invention is in reducing UST defects in steel sheets.

〔Effect of the invention〕

As mentioned above, the present invention effectively reduces UST defects in steel sheets by combining low-speed rolling and high-reduction rolling during finish rolling, so even extremely thick steel sheets, such as steel sheets with a diameter of 80 mm or more, can be continuously cast. Since it can be manufactured more easily than slabs, its industrial value is extremely high.

[Brief explanation of the drawings] Figure 1 is a diagram showing the center porosity before and after rolling (a
) is before rolling (b) is after rolling, Figure 2 is a diagram showing the relationship between rolling shape ratio (Mi) and porosity pressure stress function f (Mi), Figure 3 is rolling speed (Vi) Figure 4 shows the relationship between the plate thickness and rolling shape ratio between each pass in the case of general rolling (finish rolling temperature 900°C). Figure 5 is a diagram showing the relationship between plate thickness and rolling shape ratio between each pass in the case of controlled rolling (finish rolling temperature 750°C). Figure 6 is the number of UST defects showing the effects of the present invention and the conventional method. FIG. 7 is a diagram showing the relationship between rolling speed and residual porosity thickness ratio. Fig. 2 Fig. 1 Fig. Rolling speed mm/Sec Estimated plate thickness (mm) Fig. Plate thickness (mm) Fig.

Claims (2)

[Claims] (1) In a method for producing extra-thick steel plates, in which a continuous casting slab is tentered in a rough rolling process and further rolled to the product thickness in a finish rolling process, the rolling speed is set at 200 to 350 mm/min in the finish rolling process. A method for producing an extra-thick steel plate with excellent internal properties, characterized by rolling multiple passes at sec. (2) In a method for producing extra-thick steel plates, in which a continuous casting slab is tentered in a rough rolling process and further rolled to the product thickness in a finish rolling process, the rolling speed is set at 200 to 350 mm/min in the finish rolling process. sec for multiple passes, and all passes are rolled at a rolling shape ratio of 0.
A method for manufacturing an extra-thick steel plate with excellent internal properties, characterized by applying a reduction in the thickness direction at a thickness of 5 or more. ▲There are mathematical formulas, chemical formulas, tables, etc.▼ M_i: Rolling shape ratio ld: Roll projected contact length (mm) hm: Average sheet thickness in the roll gap (mm) R: Roll diameter (mm) h_i: Sheet thickness (i = 0, 1, 2...) (h_0: Plate thickness before rolling) (h_i: Plate thickness after i passes after rolling) i: Number of rolling passes