JPH0255606A - Manufacture of very thick steel plate of excellent internal quality - Google Patents

Abstract

PURPOSE:To obtain a very thick steel plate of excellent internal quality by drafting a part between a liquid phase line crater top and a solid phase line crater top of a billet by a drafting amount in a specific amount range in constant drawing stages and performing plural rolling passes at speeds in a specific rolling speed range in finish rolling stages. 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 constant drawing stages, a part between a liquid phase line crater top and a solid phase line crater top is continuously rolled by a drafting amount in the range of 0.5-2.0mm/min. In finish rolling stages, plural rolling passes are performed at a speed in the range of 200-350mm/sec. Further, the stock is drafted by a rolling shape ratio of >=0.5 in the billet thickness direction. An equivalent diameter of a center porosity d0 and a billet thickness h0 before rolling are changed to a diameter dk and a billet thickness hk after K passes to show a pressure welding degree by plastic deformation of the stock. Hence, UST defects in the steel plate are effectively reduced and even a very thick steel plate is easily manufactured from a continuously cast billet.

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 plate thickness of 80 fins or more, in which the material to be rolled is a slab produced by a continuous casting method.

[Prior art]

Conventionally, in slabs obtained by continuous casting, the final solidification position 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. When the product plate thickness is 80 mm or more, it is impossible to sufficiently reduce the slab obtained by continuous casting, and it is impossible to manufacture a product without UST defects. For this reason, in the conventional method for producing extra-thick steel plates, as shown in Japanese Patent Application Laid-open No. 62-151201 and Japanese Patent Publication No. 62-13083, the rolled material is rolled by the steel ingot method. is used.

The problem with such conventional technology 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 There is a "thick plate rolling equipment" that is equipped with a casting press that rolls down in the direction of the continuous casting method.
This suggests that it is impossible to manufacture extra-thick steel plates without using a casting press when UST defects are taken into consideration.

The problem with such casting press equipment is that it requires more expensive equipment than rolling equipment with rolls, and the cost of utilities such as electric power is also high.

In order to solve the above-mentioned problems of the conventional technology, as a method that does not require capital investment, as shown in Japanese Patent Application Laid-open No. 61238404, a steel material with a shape with a temperature difference of 400°C or more between the surface and the center is proposed. ratio (lengthwise effective contact length (fin)
There is a "hot working method for steel materials" which is characterized by applying reduction in the thickness direction and/or in the middle direction when the thickness (fin) is 0.5 or more. On the other hand, it has the advantage of not being necessary, but “4
(1) In order to create a temperature difference of 400°C or more between the surface and center of the slab obtained by continuous casting, water cooling is required on the surface of the slab. As a result, the loss in unit heat consumption becomes large.

(2) In order to achieve a large reduction with a shape ratio of 0.5 or more when the surface of the slab is 400°C or more lower than the center, the rolling capacity of the rolling mill must be several times higher than that of normal rolling. It takes twice as much.

Therefore, a large capital investment is required to increase the capacity of the rolling mill.

On the other hand, unlike the technology that compresses center porosity in the rolling process, there is a technology for suppressing center segregation and center porosity in the continuous casting process.
As shown in Publication No. 62, "In the drawing process following the secondary cooling zone in continuous casting of molten metal, the liquidus crater tip and the solidus crater tip of the slab are separated by one or more pairs of reduction rolls. There is a "continuous casting method" which is characterized in that the rolling reduction rate per pair of rolls is 1.5% or less during the steady drawing process between the two rolls.

The feature of this conventional technology is that roll reduction is performed before and after complete solidification to prevent the concentrated molten steel that has formed between the dendrite branches at the solidification interface at the crater tip from moving, thereby suppressing the occurrence of center segregation and center porosity. It's there. The problem with this technology is that even if center segregation is reduced to some extent, the occurrence of center porosity cannot be suppressed at all. The rolls of continuous casting equipment are designed to suppress bulging between the rolls due to static pressure of molten steel.
Especially in curved continuous casting equipment, the roll diameter can be up to 400 mm.
~500mm can only be obtained. As described above, since the mouth in continuous casting equipment is much smaller than the roll used in the rolling process, there is not enough crimping capacity to completely crimp the center porosity in the continuous casting process.

[Problem to be solved by the invention]

The present invention solves the above-mentioned problems of the prior art, ie, the high equipment costs and the insufficiency in suppressing the occurrence of center porosity, in a method for manufacturing extra-thick slabs by continuous casting.

[Means for Solving the Problems] The present invention advantageously solves the problems of the prior art described above and has the following features.

That is, in a method for producing an extra-thick steel plate, in which a slab is manufactured using continuous casting equipment, the slab is then subjected to tenter rolling in a rough rolling process, and further rolled to the product thickness in a finishing rolling process, the above-mentioned continuous casting equipment In the steady drawing process, the area between the liquidus crater tip and the solidus crater tip of the slab is reduced by 10.
5-2. It is rolled down continuously in the range of Ow/min, and the rolling speed is set at 200 to 350/s in the above finish rolling process.
It is characterized by multiple pass rolling within the range of ec, and further,
The method is characterized in that all of the pass rolling in the finish rolling step is performed by applying reduction 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.

When producing slabs in curved continuous casting equipment having vertical sections, the area where the slabs completely solidify is generally the horizontal roll zone. FIG. 1 shows the roll spacing in the region before and after complete solidification of the slab in the above-mentioned casting equipment. FIG. 1(a) shows the area before and after complete solidification of the slab. The solid lines 1.2 are the in-phase line and the liquidus line, respectively, when a light pressure according to the present invention is applied.

Dotted lines 3 and 4 are the solidus line and liquidus line, respectively, when no light pressure is applied. FIG. 1(b) shows the change in the distance between the rolls before and after the slab completely solidifies.

In the present invention, the solid phase end point (B1) is reached before the liquid phase end point (A1).
Until the end, light reduction with a reduction amount of 0.5 to 2.0 + u/min is applied to completely suppress the flow of molten steel before the complete solidification region. Note that Ax and Bz are the liquid phase end point and the solid phase end point when light reduction is 0.1 to 0.2 min/min, taking into account only the influence of solidification shrinkage without applying light reduction.

Under the above light reduction conditions, if the reduction amount is less than 0.5 m/min, it will not be possible to suppress the flow of molten steel and center segregation and center porosity will not be reduced.
/ minute is the lower limit. On the other hand, if a light reduction is performed in which the amount of reduction exceeds 2.0 mm for 1 minute, the normal roll diameter, that is, 300 to 5
In the case of approximately 00*φ, roll bending occurs because there is no rolling capacity compared to the roll rigidity. This causes molten steel to flow in the width direction of the slab, increasing center segregation and center porosity. In addition, in order to increase the rigidity of the roll, it is desirable to increase the strength by using split rolls, but even if the strength is increased, 2. Under light pressure exceeding OmmZ, the special public
As shown in Publication No. 862, internal cracks occur frequently. Therefore, the upper limit of the amount of reduction in light reduction according to the present invention is set to 2.0 m/min.

As mentioned above, in the continuous casting process, internal cracks are likely to occur in the slab, and for this reason, it is not possible to achieve a large reduction before the completely solidified region.As for the quality of the slab, center segregation can be suppressed, but center porosity is Cannot be completely deleted. When light reduction is applied to a specific part of a slab as in the present invention, the equivalent diameter of the center porosity (approximately 3 to 4 msφ) is halved compared to when no light reduction is applied, but the equivalent diameter is still 1
There is a center porosity of about 2 dragons φ. Unless a large reduction is applied to the slab during the rolling process, such center porosity will remain as a defect inside the product steel sheet, and specifically, it will be rejected as a LIST defect.

In order to eliminate such drawbacks, the present invention performs the following rolling in the finish rolling process.

First, after producing a slab (for example, slab thickness of about 300 mm) as described above, the slab is heated directly or in a heating furnace and then sent to a rough rolling mill, where it is rolled. . Rolling by this rough rolling mill is mainly for tentering rolling, and multiple pass rolling is performed, and the thickness of the rolled material is the product plate thickness (e.g. 1
00-150 mm) approximately 1.5-2 times (e.g. 200 mm)
~250mm).

Thereafter, the product is rolled to the product thickness by multiple passes in a finish rolling mill, and in the present invention, low-speed rolling is performed with the rotational speed of the rolls in the range of 200 to 350 mm/sec.

That is, in the above rolling process, as multiple bus rolling is performed, it is most desirable that the center porosity gradually decreases until it is crimped, but as the center porosity gradually decreases, the area around the center porosity is undergoes plastic deformation under the load of The progress of this plastic deformation 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 this plastic deformation progresses as the contact time between the rolled material and the rolls increases. In this way, low-speed rolling is extremely effective as a method of increasing the contact time between the rolled material and the rolls in order to efficiently compress the center porosity while reducing it without increasing the rolling capacity of the rolls.

The present invention performs such low-speed rolling to compress the center porosity within an allowable range, and the reason why the rotational speed of the rolling rolls is specified as described above is as follows.

That is, when the rotational speed of the roll is less than 200 n/sec, the contact time between the rolled material and the roll increases, and as a result, with a general rolling roll, heat cracks and rough skin are likely to occur on the roll itself due to heat load. Therefore, the lower limit of the roll rotation speed was set to 200 am/sec.

In addition, the rotation speed of the rolls of the finishing rolling mill was set to 350 mm/
If the product is rolled at a speed exceeding 1.0 sec, UST defects will occur in the product even if the shape ratio is large (approximately 1 m3), so the upper limit of the rotational speed of the finish rolling mill roll is set at 350 nm/sec.

In order to carry out such low-speed rolling, the bank-up roll bearings of finishing rolling mills have to have low load-bearing performance, because in the case of general movoile oil film bearings, it is difficult to form an oil film on the bearings, causing oil film seizure. Good rolling bearings are best.

Next, in order to further improve the internal quality of the slab, in addition to the method of the present invention described above, it is desirable to apply large reduction rolling to the rolling mill using a finishing mill.

The large reduction rolling will be explained below.

In the present invention, a large reduction is applied to a slab that has been lightly reduced during casting under conditions of low speed rolling, and the rolling shape ratio M4 is limited as an index of large reduction rolling.

Shape ratio M for each pass is M j = Rd / hm M, : Rolling shape ratio 1d : Roll projected contact length (11) hm : In-hospital average plate thickness between rolls (l-) R : Roll diameter (1 m) h,: Plate thickness (j=0.1, 2...) (ho: Rolled surface plate thickness) (h,: Plate thickness after j passes after rolling) j: Number of rolling buses, and this rolling shape If all of the ratios MJ are less than 0.5, the center porosity will decrease in proportion to the decrease in plate thickness in the finish rolling process, but crimp will not occur even if the number of rolling steps is increased. This phenomenon is caused by tensile stress acting on the center of the steel material and no compressive stress acting at all, so even if the number of rolling steps is increased, the center porosity cannot be crimped. Therefore, the lower limit of the rolling shape ratio Mj is set to 0.5. On the other hand, the upper limit of the rolling shape ratio is the slab thickness that can be manufactured using the current continuous casting method, which is 300.
~350 Dragon level, product thickness of extra-thick steel plate 100-200m
Considering m, it is about 3.

If the equivalent diameter of the center porosity before rolling of a continuous casting slab is d, and the equivalent diameter of the center porosity after K-pass rolling is dl, then dK/d0 is: f (Mj) : Volocity compressive stress function g ( Vj
); Porosity crimping speed influence coefficient j; 1.・・・・・・
・K. However, Mj<0.5, Vj ≧0.5m/se
When c, equation (2) becomes dll/d,, #hk/h0 (3). FIG. 2 is a diagram showing the center porosity shape before and after rolling. FIG. 2(a) shows the center porosity before rolling, and has an equidistant diameter d0 and a slab thickness h0. Figure 2 (b
) indicates the center porosity after bus rolling, and δ is the equidistant diameter dk, and due to low speed rolling and large reduction rolling, the contact length and contact time between the roll and the rolled material are increased. This indicates the degree of crimping progress due to plastic deformation of the material caused by the bending.

Normally, when the rolling speed is 0.5 m/sec or higher, δ=0, and as shown in equation (3), the isometric diameter ratio dk/d0 of the center porosity before and after rolling is the thickness ratio of the rolled material before and after rolling. , /h, so theoretically the center porosity cannot be eliminated by compression. The content of equation (4) regarding the crimp effect of center porosity will be explained.

Figure 3 shows the rolling shape ratio Mj and porosity compressive stress function f
(Mj), both of which are expressed by the following equation (
5).

f (Mj) = aMj2+ bMj+ c
(5) From the above equation, the porosity compressive stress function f (M
j) is an increasing function of the rolling shape ratio Mj, the rolling reduction in one pass is small, and in the region of Mj < 0.4 a, the porosity compression effect due to the rolling shape ratio is very small, and f (Mj) = 0.

FIG. 4 is a diagram showing the relationship between the rolling speed Vj and the porosity crimping speed influence coefficient gm), both of which are related by equation (6).

g(Vj)= +e (6)M
j The porosity thickness decreases as the strain rate decreases, and the bonding of the porosity inner surface increases as the compression time increases.

That is, the porosity crimping speed influence coefficient g(Vj) is expressed as a decreasing function of the rolling speed Vj. Moreover, this low speed effect g (Vj) promotes porosity crimping as a synergistic effect with the large rolling effect f (M·), and the low speed effect is small in a region where the rolling shape ratio is small.

FIG. 8 shows the relationship between rolling speed and residual porosity thickness ratio d, /d0 when the shape ratio is 0.5. Rolling speed 0.35m
/sec or less, the residual porosity thickness dk becomes almost 0.

In addition, when low-speed finish rolling is performed as in the present invention, compared to the conventional example of normal finish rolling, it is possible to obtain a larger rolled shape ratio per pass, which combines with good rolling reduction during casting and low-speed rolling effects. , it is possible to obtain thick steel plates with excellent internal properties.

Figure 5 (General rolling: Finish rolling temperature 900℃), Figure 6 (
Controlled rolling (finish rolling temperature: 750°C) indicates the relationship between the plate thickness between each rolling pass and the rolling shape ratio, and compared to conventional rolling (rolling speed of 2000 mm/sec), low-speed rolling (rolling speed of 300 + m/sec) ) indicates that it is possible to obtain a rolled shape ratio of approximately 0.1 increase.

In general rolling, with a slab thickness of 300 fins, the maximum product thickness is 2
00mm can be manufactured. The rolling conditions shown in FIG. 5 are as follows.

Controlled rolling is used in the production of steel sheets that require low-temperature toughness, such as marine structural materials.The low rolling temperature is disadvantageous for large reduction rolling, but low speed large reduction rolling can reduce the product thickness to 10.
It is possible to manufacture a steel plate with a thickness of 0 m5 and excellent internal properties. The rolling conditions shown in FIG. 6 are as follows.

〔Example〕

In the process of drawing slabs with a slab thickness of 3001fi using continuous casting equipment, light reduction is continuously applied between the liquidus crater tip and solidus crater tip of the slab at the reduction radius shown in the table below. , Next) ■ In rolling, perform extrusion rolling,
After tentering, finish rolling was performed at a temperature of 750° C. and a rolling speed shown in the table below. Product size is 100 meters x 2500
It was m5.

Figure 1 shows the internal properties of each material. This figure clearly shows how the synergistic effect of the combination of light reduction during continuous casting and low speed rolling during finish rolling of the method of the present invention is effective in reducing UST defects in the steel sheet.

〔Effect of the invention〕

As mentioned above, the present invention extremely effectively reduces UST defects in steel sheets by combining light reduction during continuous casting and low speed rolling during finish rolling. Since steel plates can be manufactured more easily than continuously cast slabs, their industrial value is extremely high.

[Brief explanation of the drawing]

Figure 1 is a state diagram (a) of the slab according to the present invention before and after the complete 3-quasi-solid region, and (b) a diagram showing the roll spacing. Figure 2 is a diagram showing the center porosity before and after rolling, and (a) is a diagram showing the center porosity before and after rolling. Before (b
) shows the result after rolling, and Figure 3 shows the rolled shape ratio (Mj)
Figure 4 is a diagram showing the relationship between rolling speed (Vj) and porosity crimp speed influence coefficient g (Vj), Figure 5 is a diagram showing the relationship between rolling speed (Vj) and porosity compression stress function f (Mj), and Figure 5 is a diagram showing the relationship between rolling speed (Vj) and porosity compression stress function g (Vj). Figure 6 shows the relationship between the plate thickness between each pass and the rolled shape ratio in the case of controlled rolling (finish rolling temperature of 750°C). FIG. 7 is a histogram of the number of UST defects showing the effects of the present invention and the conventional method, and FIG. 8 is a diagram showing the relationship between rolling speed and residual porosity thickness ratio.

Claims (2)

[Claims] (1) In the method of manufacturing an extra-thick steel plate, in which a slab is manufactured using continuous casting equipment, the slab is then subjected to tenter rolling in a rough rolling process, and further rolled to the product thickness in a finishing rolling process, the above-mentioned continuous casting In the steady drawing process of the equipment, the reduction amount between the liquidus crater tip and the solidus crater tip of the slab is 0.
Continuous rolling is carried out in the range of 5 to 2.0 mm/min, and the rolling speed is set to 200 to 350 mm in the above finish rolling process.
A method for manufacturing an extra-thick steel plate with excellent internal properties, characterized by rolling multiple passes within a range of /sec. (2) In the method of manufacturing an extra-thick steel plate, in which a slab is manufactured using continuous casting equipment, the slab is then subjected to tenter rolling in a rough rolling process, and further rolled to the product thickness in a finish rolling process, the continuous casting described above. In the steady drawing process of the equipment, the reduction amount between the liquidus crater tip and the solidus crater tip of the slab is 0.
Continuous rolling is carried out in the range of 5 to 2.0 mm/min, and the rolling speed is set to 200 to 350 mm in the above finish rolling process.
/sec, and all passes are rolled in the thickness direction of the slab with a rolling shape ratio of 0.5 or more based on the following formula. Production method. ▲There are mathematical formulas, chemical formulas, tables, etc.▼ However, M_j: Rolling shape ratio ld: Roll projected contact length (mm) hm: Average sheet thickness in the roll gap (mm) R: Roll diameter (mm) h_j: Sheet thickness ( j=0, 1, 2,...) (h_0: Thickness before rolling) (h_j: Thickness after j passes after rolling) j: Number of rolling passes